Thanks for your thoughts, Joe. Your points are noted and well taken. I am going to send the stove to be checked over anyway, I decided, because I am a little mistrusting of the combustion fan. That one for sure has to work the way it is designed. I am a little leery also of tampering with the circuit to the blower unit. The one check I made on it showed the voltage at one of the motors (there are two motors on the blower fan) to be 8.2v, and I suspect that the supply voltage might vary as controlled by the circuit board electronics to change the fan speed. (That's why I was planning to use a solid state relay. It will work with a signal input of 3 to 32 vdc.)
So, if the stove gets worked over, the blower fan will be repaired or upgraded while it's there. Then I will probably just use the existing blower to push the air into the planned collector and ducting. I still think that would be a cool (or warm!) feature to have. I could even put a shuttle valve in the duct to direct the air into the v-berth for a quick warm up.
The ducting run will be completely concealed, and it will be short enough that I don't foresee the need to insulate it beyond the capabilities of the hose itself. Any heat that does escape will be inside the cabin, anyhow.
Chances are, I won't add the defrost feature at all. The stove will eventually dry the interior enough to keep the windows clear by itself. I just figured that it would be handy on cold, damp mornings to get me away from the dock and out on the river sooner.
Sometimes I come up with weird ideas, you see.