Came across this while doing a little browsing. Finally someone has proved my idea works.
Basically, you just put pipe in the concrete and pump the water through it. It occured to me a few summers ago when I stepped barefoot onto asphalt on a sunny day. There's a lot of potential heat there. If you used in in pool decking, not only do you have more solar collector sq footage, but the concrete stays cool on your feet, esp. if you have darker colored crete.