Successor game rules, draft one.

[_Tristan_]

From what I know of French history, dueling per se didn't appear until the 16th century when swords got lighter.

What we had before was the "Jugement de Dieu" (God' s Judgment) in which the aggrieved party met its opponent on the field to settle their dispute before God, the will of God being expressed by the gift of victory.

Most times, this judgment was first conducted on horseback with lances and if necessary was prolonged with whatever weapons were chosen, axes, swords, maces...

I fully agree that implementing such a system could be a great addition to the game.

[Cecil XIX]

All right, here's what I got for ancillaries. This analysis doesn't include the ancillaries effects on the four basic stats or valor, if those stats end up mattering. I didn't include unique ancillaries like religious artifacts for historical persona, they can be dealt with as they occur. Since there's a limit to how many ancillaries a character can have, unlike with traits, it may be practicle to factor in them all. If nothing else, this could serve as an RP guide for people who want to write stories about the run-up to a duel.

Spoiler Alert, click show to read: 

• academic_advisor - Unless he can find you some 'fighting manuals', he won't be very useful.
• actor - Could help you get the audience on your side, otherwise useless.
• adultress/foreign/she-male - Could affect the audience, but that's it.
• alchemist - HP bonus is definitely applicable, and if we allow dueling with pistols later he'd help you get high-quality weapons.
• apothecary - HP bonus is helpful, perhaps he gives a stat boost via ye olde energy drink?
• architect - Only way this would help is if you were fighting in an arena built by this guy, and he included
  trap doors and giant blades coming from the ceiling.
• armour_custom - This is probably the most obviously beneficial ancillary in the game. Armor is important, right?
• armour_ornate - Worse than your average noble's armor, but could help get the crowd on your side. I suppose you could choose not to wear it, but then you can't choose in a real battle.
• artist - I can't see this guy being helpful at all during the battle, but if he immortalizes your victory afterward that could increase certain rewards.
• astrologer - I suppose if you believe this guy, his predictions could affect your performance.
• bard - This guy would be a real crowd pleaser, if that matters. And like the artist, it's always nice to have someone immortalizing your past victories, right?
• biographer - Again, this guy could only be helpful after the duel; always will to write of battle in the best terms.
• black_stallion - Helpful in a joust, useless outside of one.
• bodyguard - A sparing partner for better practice, or perhaps an above average champion should you choose to decline.
• brilliant_inventor - Conceivably he could invent some fancy do-hicky to surprise your opponent with.
• caravan_driver - He could bring you something useful from a distant land, if that isn't streching things.
• crooked_judge - He could do something cheesy like have your opponent arrested, but I doubt that'd be allowed in the rules.
• doctor - Having the best available personnel to give you care and a good once-over after a duel has obvious benefits, what those are depends on the rules.
• drillmaster - Undergoing an intense physical regimen is a good way to train body and mind to operate calmly and effectively in a fight.
• evil_mother-in-law - Having her in the audience, cheering on your opponent, could either be a real downer or a keen way to focus your anger.
• foodtaster - If your opponent tries to poison you before the duel, this guy could be really helpful.
• fool_brilliant - Perhaps this idiot savant has noticed something about your opponent in a moment of clarity...
• fool_usual - A warm-up act for the audience, if nothing else.
• harsh_judge - Probably not the best time to remind the audience who this man's patron is...
• herald - The closest thing you can get to entering the field of honor with your own theme song playing.
• intrepid_explorer - If you chose to join this man on his expeditions through rediculous terrain, it is child's play to move deftly through a flat arena.
• knight_beserker - A good champion to fight in your stead, and sparring with this man will definitely help you defend against your opponent's attacks.
• knight_chivalrous - Another fine champion, and a good sparing partner who will get you used to fighting someone *else* who's chivalrous.
• knight_dread - Same as a knight_chivalrous, just with dread instead of chivalry.
• knight_santiago - Again, a fine champion to fight in your stead and a good sparing partner. Perhaps his dedication to the Lord will inspire you as well?
• knight_stjohn - See 'knight_santiago'
• knight_templar - See 'knight_santiago'
• knight_teutonic - See 'knight_santiago'
• lancebearer - If you're jousting, it's good to know someone who can get the best lances.
• librarian - Perhaps some ancient sword treatise could ensure your victory...
• magician - The audience is sure to support a noble who employs a miracle worker...
• magician_pagan - But not someone who defies the word of god.
• master_mason - See 'architect'
• master_of_archers - Unless duels involve bows or crossbows in some way, this guy isn't going to be very useful.
• master_of_assassins - This is someone who can definitely teach you how to end a fight quickly. Perhaps he knows a way to disable someone without killing them?
• master_of_horse - A good horse is of great benefit in a joust, but only their.
• master_smith - Making sure your weapons and armor are of good quality is quite a boon.
• mathematician - If only he could calculate kinetic energy and the pressure at the tip of an opponent's weapon fast enough to be of help..
• mentor - Old men with experience can have all kinds of useful information.
• mercenary_captain - This guy can set you up with some sparing partners, and even find a top-class warrior to fight in your stead.
• military_engineer - Unfortunately, this guy's engineering is useless for personal combat.
• money_counter - Unless you need help getting enough money for arms and armor, this guy can sit on the sidelines.
• musician - A good choice to warm up the audience. And people will remember who his employer is!
• nosy_mother - This is not a distraction you want when you're preparing for a fight, but at least she stands between you and your enemies.
• ordinance_master - If only, if only...
• overseer - Good management can't help you in the heat of battle, not unless you're in need of a quick mob.
• pet_guarddog - Restrained or not, if he sees your opponent going for the coup-de-grace...
• physician - See 'doctor'
• priest - Having a man of god on hand can help your peace of mind. At least he can quickly administer last rites.
• quartermaster - Perhaps this man can quickly acquire high-quality arms & armor for you?
• royal_escort - Stand in, sparing partner and someone you can keep an eye out for fishy business. It's good to royalty.
• runner - Athletics are a good way to prepare for battle.
• scout - A man who's used to scouting out the enemy in preperation for battle, where ever the field lies.
• scribe_ancillary - Not much use for writing in battle, unless he can find you a book on techniques.
• shieldbearer - When your shield stand between you and the enemy's blade, you'll be glad you had a man who could get you the best shield.
• siege_engineer - See 'ordinance_master'
• soothsayer - See 'astrologer'
• spymaster - The perfect choice to find your opponent's weaknesses, as well as safeguard your own.
• swift_steed - A good horse is invaluable in a joust.
• swordbearer - The more swords you have access to, the more you can have faith in the best of the bunch.
• tax_farmer - If you need money for bribes, this guy's your man. But would you?
• torturer - No matter how badly you beat your opponent, you can't sick this guy on him. Still, he can teach you a few tricks on how to cause pain your opponent can only imagine.
• treasurer - See 'tax_farmer'
• trusty_steed - To you and your horse, a joust is just another battle.
• tutor - See 'mentor'
• veteran_warrior - Perhaps the best champion and sparring partner a nobleman could ask for.