The whole reason we are discussing recruitment during civil war is because of the fear that an unscrupulous Seneschal will pump up his pals, starve you and set his pals on you. If he is going to those lengths, don't you think he would take care to empty the merc pool in your area before declaring war?
Well, I think the problem of the mercenary pool being depleted before the civil war starts can be easily resolved by not allowing the Seneschal to recruit mercenaries, except a) for civil war purposes b) when the Senate votes legislation that allows it.

As for a Seneschal pumping up his pals, well that's IC, I think. It'll make the position of Seneschal even more important; Houses will have to take into account who they are supporting and very often, the Seneschal will be somebody neutral.

And a backstabbing Seneschal, helping one House to prepare for Civil War will only add more to the drama

In short: recruitment of mercenaries is not allowed, except when a) authorised through an Edict; b) for Civil War purposes.

As for "regular" troops: IC, as in: make sure the right Seneschalk gets elected