Yep, ya WANT a leaky Heat Source, if You are Cooking AND Heating the space. So a small heatsource, insulation good enough to reach desired Cooking Temperatures, with external surfaces that emit enough Heat for Space to be Comfortably warm, would pose a different problem than just an Oven.
Sounds a lot like a Soapstone walled Woodstove! Too hot in Summer, though. Then Ya go to the Summer Kitchen - Outdoors!