It really wouldn't be unreasonable to allow it to be applied to those spells as long as you keep one thing in mind.
Dispel Magic.
Just don't let them keep their empowered bulls strength very long. They won't want to do it again I bet, since is costs XP.
And don't forget that bonuses of the same type don't stack, so bulls strength and belt of strength don't go together too well.
--Lenient Spikey