A very handy modification is to use a water heater with an engine coolant coil in it, many marine units have them, and to install a small electric pump to pump coolant into the water heater when the engine is warm. This give lots of hot water underway, and for quite a while after. I have seen several larger boats that use this system to move heat from the generator to the hot water tank.

Running in reverse would certainly keep the blocks warm as long as the power to the hot water heater held out...

I would think that the bilge blowers would cool the engine room and thus the cabin faster than using the water heater.