Seperate thermostats in each room and or a central control center and there you go. Of course, make sure you heat that room a day or so before guests arrive because it does take a bit of time when it's really cold. But invert the whole mess. I've got to beat this concrete puzzle. Concrete equals very high embodied energy.
Thermal mass does tend to even out the temperture fluctuations which tends to be desirable.
I think that a radiant sand subfloor would be a good idea. I have installed hydronic heating systems using PB tubing in concrete slabs, on wood subfloors with concrete covering, and under wood subfloors and have thought about just embedding the tubing in sand as you have suggested. As to floor covering, I'v been in a house that had carpet over 6 mill poly over sand and you could not tell the difference over concrete. The sand was compacted first using a little water. I'll try to.