I have been thinking about a variation of this for quite a while. It makes me itch that I missed my oportunity (floor poured!) to try it by about a week.
If you have the solar water heaters already, and you are waiting to figure out the rest of it... How about a temporary set up?
Like a 5 gallon pail for expansion, and a marine (boat) live well pump? They are 12 volts, seem to move a lot of water for the power used, and pretty cheap.
Pretty much cold water out of the floor, through the pump, through the bucket, through the heaters, into the floor. A air conditioner type thermostat inside the heater could be set to say 90', so when the temp. get there it would turn on a small (high coil resistance) 12v relay that would turn on the pump.
It wouldn't be fancy. It would be cheap, at least until you get all the fancy controls, pump, expansion tank, etc.
The place I missed was going to be heated with wood/coal, in an area where it may be 40' (fire burning) 3 days, then 60' (cool- but no fire) for 3 days. I was thinking of using the floor to store some of the heat made during the cold days. Solar electric (already there) was to pump the water past the wood burner (already there). I'm not talking anything ground breaking, just more like an experiment. Everything extra would have been about $150. Just plastic and copper (by the stove) pipe, and a pump.(the PV etc. was already there) It would be useless in the winter and summer, but nice in the spring and fall. It's an Amish house (you should see his solar electric set-up!).
Any thoughts as to why this wouldn't be any good?