I may not have the best solution for the OP, but I'm interested in this topic.
Allow me to toot the horn of Softub a bit. When I was hot tub shopping and researching, I decided on Softub for power efficiency. Most hot tubs use gas or 220v electricity and are power hungry to use. Softub is quite different whereas it heats without the use of a "heater". As the 110v filter pump cycles the water, the heat from the blower motor is used to warm the water. You set the thermostat at your desired temp and the filter pump turns on as often as needed to maintain the set temp. It takes much longer to heat than a convention tub, but the energy cost is much lower. For instance, after filling with fresh water, it takes me 24 hrs to reach temp. (I set mine at 100*F.) Then it always stays hot by turning on a couple times per day. The reason I am explaining this is to show efficiency. In fact, it is so efficient that my tub has remained at 100*F for the last 1.5 years without anybody using it. (Not something I am proud of.) I don't notice any difference in my power bill.
So I'm thinking, if you hook up a Softub motor pack, you can cycle the water and add heat at will. Maybe put it on a timer a run 6 hours a day and aim for 82*F? You can probably find a used motor pack online for $100-$200.
Just a crazy thought but it might be worth some consideration.