Interesting idea. I one-off increase of AI forces in Frances vicinity might cause balancing problems, but if it's for the english provinces in France then it shouldn't be a problem. Those will be almost completely surrounded by us anyway.

I definitely like the idea of editing the king's purse for AI factions. That would be easier to implement, and I suspect the AI would also have an easier time "seeing" what is going on; it might make better use of the money.