Hey Scott,
Love your idea about shrouding the turbo and using it as your heat exchanger. I have a crown turbo kit and I currently have issues with heat from the exhaust side of the turbo "cooking" the trunk floor and carpet above. I purchased a DEI Turbo Heat Barrier kit and plan on installing it shortly. Another idea for heat would be to duct around a auxillary oil cooler with a fan like they do for the heater kits in bugs.
From what I have read, I am not sure if heater boxes on a turbo 914 are such as "hot" idea.
Reason being that you want to retain as much heat in the exhaust manifolds leading up to the turbo to reduce turbo lag and help the turbo operate more efficiently. I am planning on wrapping all the exhaust manifolds in heat tape and eventually fabbing new ones from 75-76 manifolds and having all the components ceramic coated.
Rob