ime, the hottest wheel rims i touched after a heavy prolonged braking were "quite warm", 50-ish degrees perhaps. i think there's a large gap/margin before inner tubes start becoming unsafe.
I have given this problem a LOT of thought and 50degC is plenty to cause the problems that I have witnessed first hand on too many occasions, while tandem touring fully laden.
Inner tubes if looked at like a huge number of rubber bands (rather than a tube) stretched near their limit then heated in one eighth of their circumference, will thin at the point that is warmest as the material becomes more flexible, as the cooler part of the band is trying to return to its original length with greater force than the part that is being warmed. As the material thins, it becomes even more susceptible to the heat, thins more etc, and rapidly reaches the point that tiny holes appear.
So think about the temperature difference in the length of the band rather than the total average temperature of the tube.
Maybe I am wrong, but I think, if the innertube were heated equally to 50degc within the confines of the tyre and wheel rim, it would happily continue to contain the gas, all the time the pressure does not increase over and above the recommended maxima for that tyre's size.