Interesting. I once built a small thermal hot water heater system on my kids little wading pool. It was pretty simple... just 75 feet of black pipe on the roof of the shed hooked up to a small circ pump, but it work quite well and kept the chill out of the water.
I do have a completely independent circ pump system on the hot tub to handle heat and filtration. The circ pump is a small grundfoss pump that you would see circulating hot water on a home furnace (something like 1/4 horsepower). I have this hooked up to a simple timer so I can control how much it runs in a day. In the Summer I have it running for 4 hours in the morning and 4 at night and in the Winter that gets bumped up to 3 times a day (4 hours each). I used to have it running 24/7 but found through experimentation that it really wasn't necessary, but of course that all depends on how much the tub is used and how clean people are when they enter.
At any rate, something along this line is what you're probably going to have to do with your thermal hot water system addition. You wouldn't want to run it off your main pump because you will most likely lose too much pump horsepower in circulation (which means less power to your jets). You will also need to connect it into a solar switch so that it's only on when the sun is out. And of course depending on how hot the sun is in your location, a thermostat switch may be necessary to avoid overheating the water.
