While mine hasn't overheated, I have encountered the flashing "TEMP" (temperature charge error, according to the manual p.14) within the battery level box on the screen while running with an external Talentcell battery. Removing the connection immediately resolves the issue and the level indicator returns to normal.
Reviewing the manual, it states these Operating Conditions:
Charging Li-ion battery 0 to +45°C (113F)
Operating temperature -20 to +70°C (158F)
It appears that while charging the threshold for overheating is lowered, which makes sense. My experiences with the TEMP error were during the summer, although outside ambient temperatures didn't really exceed ~80-90F, but it seems possible that even a well-ventilated rig bag could easily exceed 113F. That said, I'm not a big fan of the required 18650 battery. Having to step away for an entire set or show while leaving an external battery connected might run the risk of overheating if you aren't able to catch the TEMP error in time. I haven't ran enough tests to figure out the best solution.
Are people comfortably running for an entire show on a single 18650?