[LDiCesare]

I tested, and it's indeed possible to mod spells in-game. There are just weird behaviours on the first turn when you mod them (the spell appears to remain in the old school, but with new description and all, so you may not be able to cast it if you have the required level in the good school but not the old school. It then behaves correctly after one more turn.)
I don't know how you tag a spell as national, however, so I just tested with a #newspell.

You might even add some creatures which cannot be summoned unless the spell has become allowed. Or ban some spells from being cast: "By five votes versus three, Tartarian Gates have been permanently closed, and Titans whall remain where they are." So Tartarian Gate spell is modded out till next vote. Or maybe it summons a butterfly instead of a Tartarian...