I think it will work. I think Garry’s right about efficiency. If you can make the water pipe cross the air flow more it will help with heat transfer.
That said, our hot water in the winter comes from our wood heater. It has only 29 inches of 3/4" hard copper tubing at the top of the combustion chamber at the edge of the top of the oven box. only about 2/3 of the pipe surface is exposed to gasses at all. about 1/4 is laying against the fire brick/oven. And the rest is on the leeward side of the pipe & not exposed to much direct hot gas contact. Also, It is caked with at least 1/2" of creosote/soot/tar/etc. And we have all the hot water we need. The pipe comes in and goes back out. One side to the top of an electric water heater, the other to the bottom. All that to say that it doesn’t seem to take much efficiency to get all the hot water that we need.
But if you want it to be more efficient, use a piece of copper tubing and make a coil that is slightly funnel shaped or bend it back and forth through the hot gas flow path.
I like the spinner. Have to make the hot gas molecules hit the pipe to make heat transfer. So maybe moving the gases to the pipe is as good as putting the pipe in front of the gases. In my experience, using the pipe (coil or similar) to make the spinner gets more heat transfered faster.
I think it is true that water moving inside the pipe and making more water molecules contact the inner surface of the pipe will increase efficiency of heat transfer and thereby reduce the need for surface area.
Also, agree that steam doesn’t need to make us shiver. Just need a little respect and care. I wish we could bring back a lot of steam tech, water pumps, food driers, engines, etc,… Especially for development settings where petroleum is not as affordable…