hot tub for solar application

Jerry, there are probably some accessible plumbing connections near the heater itself where you could tie into, but I also think that messing with the unit would void it's warranty especially if you plan to connect SHW panels directly to the tub and it's plumbing. The temperature may be difficult to maintain and the panels could get the water too hot for both you and your tub.
  The only way that I think would be safe would be to attach the SHW panels to the drain of the tub and return from the panels with a hose directly into the tub. Then you avoid using any of the unit's internal plumbing. The main concern is operation of the SHW loop. It would either have to be monitored manually or else incorporate a differential controller somehow to limit the hot water temperature.