I have the rear facing seats and they get used with my two kids aged 8 and 6 all time in the Toronto-area. Yes, we did get some extra tint to cut the heat. So far the kids are still interested in riding in the back, and we have done quite a few trips longer than 3 hours with them in the back. I also have a plug-in fan back there for extra air movement.
Two things that play out in the back seat that have nothing to do with heat, or tint, is the fact that the two kids sometimes like a little time apart from each other so one will come to the back seat for a break. The other factor is that we do these 3 hours+ drives with the two kids and the dog (medium size Shepard) in the back with them (dog lies on the floor at their feet). They love being with the dog. So, maybe the solution here is for everyone with rear-facing seats...is to also also get a dog (sorry about the fur on your new car!).
Or to move out of the south where its either high humidty, high heat (110+), or both.