The biggest issue in regular D20 is anytime you perform a coup de grace there's a 10 + damage taken save DC vs dying, so even if you took no damage from the attack (it actually healed you) you've still got a chance of suddenly pushing daisies (I guess this is the material plane equivalent of vacationing on the positive energy plane or something). I actually wrote a note into d20 legends noting that if the attack deals no damage (using the example of a healing ability being used to make the coup de grace) that they don't have to test vs dying.
Of course, some healing abilities may have no attack, or even rolls involved (the psionic ability natural healing which heals X Hp * Y PP, or things like lesser vigor or infernal healing spring to mind) so those wouldn't be applicable for those reasons.
Personally I have no problem with someone doing that. In fact, it might even create a great reason to pre-cast spells like cure light wounds and hold the charge if you intend to be healing in combat. That also has the side benefit of making healing a bit more proactive since you are essentially reading a "big heal" and then using it when someone gets clobbered. You'd need to be careful of positioning though as you'd still provoke from those who threatened you (fortunately if you're behind your target they provide soft cover which prevents AoOs).
EDIT: Also yes, as best as I can tell, you could use held charged spells with vital strike. That's an interesting thought and would definitely open some doors to launching some big heals or hurts using those feats. Spells tend to have a pretty solid pile of dice associated with them so multiplying those would be really nice. It might also be a tactic that a Magus might be interested in as well since vital striking 5d6 electricity damage can make for some pretty big booms on the cheap.