Perhaps just make healthstone healing an aura effect in a small range? Since auras exist in the game now, I think that it would make a lot of sense.
Though, I don't know how you feel about people being able to easily heal while in combat. If that's a problem, then it might be fair to give Healthstones a 'Shared Life' type effect, so that they have a pool of health to distribute to nearby allies that has to regenerate over time. That would be a very elegant solution imho. But, that's just one solution. *shrug*