Good idea, but it still wouldn't help Riza. Mustang has no clue how to use the stone to heal her, but the doctor, since he's a doctor/alchemist, probably can.
I'm also a little perplexed about the whole human transmutation and sacrifices thing. Maybe quite a few things need to go into the mixture. For the plots sake, lets just assume that normal alchemists can't really perform it, and even if they could, they wouldn't survive it. Plus something else explaining why Kimbley couldn't do it.