Hot water from a mass heater (dragonheater) -- another take

I want to steal some heat for water, but instead of taking it from the firebox, I want to take it from the mass. I don't expect it to be hot enough for showers or dishwashing. I know that stealing heat from the firebox can impact efficiency but how about stealing from the mass? I want to put pipes near the skin of the heater so that boiling point is never approached. By near the skin, I mean I want to have it just below a layer of handmade tiles which will be the exterior. I'm thinking of having the choice of circulating the water to steal heat, or leaving it still so that it just acts as a part of the mass.
What issues would you expect to see in this situation? Tell me why this is a bad idea.

Guess I will try donkey's stove forum for this... I'll let you know if I get any feedback there