I am interested it also!
I posted it already in some other topic but it got no attention there.
My idea is here, but no exact answers:
Basically you need to wrap copper pipe around exhaust and coolant flows through that spiral? How many wraps around exhaust? How much is it possible to make warmup time quicker? At least in normal driving (after warmup) I think that this extra heat is not a problem - your radiator should remove that heat.
Of course it woulld be better to stop that extra heating after warmup but it is not taht simple as solenoid valve - I would not want standing still coolant wrapped around exhaust - probably it starts to boil and make bad things.
Meelis