I like the idea of giving it to straight class wizards for free as written. Perhaps at 6th,12th and 18th levels? The idea being to give another admittedly small benefit for not multi/prestige classing.
I like that idea a lot. I could simply make spell mastery a class feature, and then add the FR feat Signature Spell (which lets you spontaneously cast one of your spell mastery spells) to the wizard bonus feat list. I think that would work nicely.
