I've never thought of doing this. It may be a possibility that this may work. My reasoning is because in my business, (vacuum heat treat furnace production) we use finned copper tube heat exchangers to cool the inert gas used during the quench phase of the heat treat cycle. On our most recent furnace, we are using 6 circuits of 2" finned copper tube to quickly quench from 1870F.
I'm pretty sure that replacing the inert gas with a different substance in a different phase (wort) would work as well.
However, I may be wrong, but aren't baseboard heaters aluminum? If they are brazed finned copper tube, my next question would be how you plan on fitting enough length into the wort for it to be efficient? I think you mentioned soldering a return bend onto two pieces? Do you think that would be enough?
Ok, ok enough with the questions! Do the experiment and report back! You might be onto something!