Gee, I am sorry, but we just don’t know for sure. That is, generally we think it is OK, but we have not tested it. From what I know about, floor heating systems are generally slow and gentle to heat and cool. Temperatures are not extreme and therefore thermal shock should not be an issue. If you install over an anti-fracture membrane, I believe you will minimize the potential of movement of the substrate to cause cracks to mirror into the surface. A good polymer-modified thinset will increase flexibility and adhesion; so I can’t think of a good reason why you should not do it.