Find some lord with an empty estate, and send him an OOC about the army thing, and see if he'll agree to let you take an estate long enough for him to remove you from the army. As long as you let him know before hand that you're just trying to work around this unfortunate limitation, it shouldn't be any big deal.
this is one thing I don't like, it's ok to meta to get around some situations a character should be able to do and can't, but not ok for other situations.
