[Sombre]

True, but no-one has shown any interest in modding the indy spell list, while numerous modders have used the copy and overwrite existing spells to create custom #onebattlespells method. If the indy spell list uses IDs then the overwritten #onebattlespell targets will end up being cast by indies.

I'm assuming you're familiar with the #onebattlespell copy and overwrite workaround here, if you aren't I can elaborate. It's evident in mods like Bretonnia and Ogre Kingdoms.