I was thinking - it's hard to explain to people that at higher difficulties base regen amounts are different. But it needn't be so. If you have -Magic or -Health regen, you already get a negative value in those boxes on your character sheet. Instead of changing the base regen at different levels, why not just give starting penalties? That makes it very similar, conceptually, to nightmare/hell difficulty in Diablo. Tell people that they get 1 health and 1 mana back every 7 turns, then just put +/- values to start, instead of changing those base rates at different difficulty levels. This changes nothing mechanically - it just makes those values more clear to players. I've also thought that a permanent regen icon might be nice - just the way some spells with upkeep give a buff icon - you could put up one with a timer which grants base regen. Again, this doesn't actually change anything - it just makes the regeneration explicit instead of an invisible hard-coded value.
The mouseover should be clear enough, ne? Not that your idea is a bad one, but the clarity you're looking for already exists pretty explicitly, just in a slightly odd place.
You could also change the current display into "number of turns needed to regen 1 mana" and then have bonuses on equipment decrease that number instead of increasing it.
