I do believe there are people that would pay you money for an option to heal all their units if there's enough gold to do so. You might even be able to make it an incentive to level if you put some kind of requirement on it (middle to end game players will virtually always have enough gold). I know I'd pay for it and I suspect it would be quite popular with all the other crazy people like me who get lost in a chat and go to fight and find they forgot to heal.
Dren