Please note though the original poster is not saying he/she is using a heat rock. They are simply asking about a rock style enclosure and how it may retain heat from the overhead heating source.

To the OP: The only way to truly know how any hide deals with heat is to monitor it. Grab an Acu-Rite and get some temps from the probe. You can place it for a day or two on top of the hide to get some reads, then tuck it inside the hide to get readings there as well. The other option is a temp gun (a PE-1 or PE-2 should do the trick nicely).