OR the hability yo have female generals . It's unrealistic but it happened few times in history around the world.
Perhaps an event that would force a female marshall into position for as long as she can remain? Would allow you to have your Joan without breaking the game.
I stand by my assertion that Joan of Arc was more of a morale figurehead than an actual military leader.
And she certainly wasn't a marshal in the game's terms, ie, military leader of the entire realm.
You could probably mod it so that a specific trait would make them eligible to be Marshall quite easily.
Whether you could make them eligible to command an army, I'm not so sure.
You can easily mod it into the game because that was a question I asked earlier yesterday. You just delete the line in the common folder, titles.txt file that says something like allow_females = no and they you can have male or females as Marshalls. Works for all the other council members as well but can lead to exploits of 20 plus in all council positions if you abuse it.
Are you under C:/Program files in vista/windows 7?
joan_darc = {
land_morale = 0.25
icon = 13
}