Jack,

This probably won't be much help, but maybe a form of magic resistance will pop up in some of the upcoming books like Magic Box, Monster Smackdown, or maybe even Beyond Human....

Other than that, I think the option of a negative modifier to the casting roll is easier to apply. Especially if it will affect non-divine healing and other such spells. You could probably make Quality out of it, then make it part of the racial Quality.
Join the Buffy Rpg Mailing list:

http://groups.yahoo.com/group/btvsrpg-eden/

Join the Terra Primate RPG list:

http://groups.yahoo.com/group/TerraPrimate/