Is it OK to install an on/off switch inside the living space to control the thermostat/humidistat control box of the power fan?

Here is some information about adding an on/off switch to a power fan.

Yes, customers can add an inline to interrupt power going to the unit, but it must be done on the line side before it comes to the thermostat/humidistat enclosure. This will preserve the unit's assembled nature and keep the UL listing.