custom battles, use your MP version of EB for the proper units.

You play the AI, or you could arrange someone to play the otherguy online. Depending on who wins, you disband units. So if 3 units rout on side A and 2 rout on side b you disband those 2 units on the campaign map. if side A kills B's general you kill that guy off thru console on the campaign map. You just gotta do all the campaign map movement yourself.
You can do this for any faction to RP civil wars.

This way you can reinact Julius Caesar and Pompey Magnus' civil war by the battle and manoeuvre!

i dont rp sieges, field battles are much easier to do.