A friend of mine put radiant under his pine floors using lightweight concrete. He put t&g plywood over the floor joists, then used 1"x2" furring strips about 16" apart. After running the tubing, he used the furring strips as a screed and filled with what I believe is called gypcrete. He then put 1" wide pine flooring on top of it. Obviously you can replace the plywood with the "car-siding". Is that like a v-groove type of ceiling board? I'm not familiar with the other products you've mentioned, but I think having the thermal mass of the gypcrete is good for radiant heat.
