Giving Resistance mana regen is a really good idea that benefits many different builds, but locking this benefit behind 100 RES makes it unrealistic for most casual builds who would be better served placing those points elsewhere.
My suggestion would be to make each point of RES restore 0.01% of the players maximum mana per second. This would encourage people to invest at least a little bit, and balances itself among different playstyles. High INT players will regenerate more with less investment because of their higher maximum mana, while low INT characters would need to invest more, which would benefit a melee build more than a caster build because of the status effect resistance.
Thoughts?