a copper pipe cheminea is what this sounds like. Let me know how it all turns out. The water flow will certainly keep the copper pipe wall temperature down. Even if it reached a boil, which it won't if you keep the flow coming. A birchbark vessel doesn't burn over an open fire, if it is filled with water or soup or broth.
See http://www.terrylove.com/forums/showthread.php?t=13761 for a funny look at how to get low cost copper that you don't need to solder.
Getting the heated water to go downhill into the pool is the big task. Unless your cheminea is located downhill from your pool, and you set up a closed loop so there won't be any siphon effect.
Sounds like you have an unlimited supply of free dry firewood. What about using solar energy? That will be free until they tax photon consumption.