Check out the Val-du-loup site ( Val-du-Loup - A Medieval Setting for Chaosium's Basic Roleplaying System BRP ). It has a lot of good stuff for historical BRP campaigns, including several new professions (including variations of knight).
I've also been working on a historical campaign set in the Crusader States.
De Gesta Francorum BRP Campaign . Not a lot of game rule stuff there, but some setting info.