A while back, I created a small hack for FE6 that replaces every promotional item in the game with a Master Seal, which can promote any unpromoted unit regardless of class (except Roy).
That's right, now you can use any combination of units you want, without having to worry about late promotions! Download!
RHDN Page
Apply on a clean Japanese ROM. The English translation made by Gringe is also applied with the patch.