Would it be OK to install an ON/OFF switch to override the thermostat on Air Vent power fans? �Does this affect the UL rating?

Installing an on/off switch to override the power fan thermostat is not recommended.

This will affect the UL listing (Underwriters Laboratory). The only way to manually control the fan right now is to adjust the temperature or humidity setting or adjusting the settings to where the fan will always run and installing an interrupt switch in the power supply to the fan.