Actually doing the emote /sit starts up the regeneration immediately for some reason (I'm not kidding).

As for the OP. It's not a bad idea really, using your weapon as a medium to channel your energy and enhancing your casts sounds logical and it would give mage jobs a bit more dynamic where you can cast for the bonus, or stay passive to regenerate MP between casts. You could go a bit further and make it where being in active mode gives a bonus to all mage related stats healing, magic attack, enfeebling, enhancing potency, and magic accuracy. And why limit it to just mages, BRDs and PLDs would greatly benefit from such a bonus too since they're always active during combat.