unless you are out there adjusting and cleaning your pool every day or 2, its going to be more work. I'm not trying to discourage you. You should just try it first on a plant like a tomato that you don't mind tossing out before you commit to fruit trees.
If you get lazy and neglect your dirt trees, they won't really care much. If you neglect a hydro setup and PH and TDS get out of wack, the plants decline pretty quickly.
If your main objective is to heat the soil, invest in some heating pads. And also try working with some heat batteries. Get creative, use something with lots of mass to warm up during the day and then it will release the heat out at night. You can even bury air lines in the ground a few feet and circulate air through the ground during the day to heat it up and then it will release the heat at night.
Even heating up a large volume of water and then using it as a heat battery. If you live in zone 6, you need a heater anyway on your greenhouse right? You must have something already? I would guess its pretty cold there this time of year.