Bathroom in-wall exhaust fan with dimmer?
This kind of fans usually use a shaded-pole motor. From wikipedia:


  they are compatible with TRIAC-based variable-speed controls, which often are used with fans


So it appears the fan should be fine.

But there is some risk to the dimmer due to voltage spikes generated by the inductive load. You could try to find a dimmer switch that is rated to control inductive loads, or you could just try it out. A typical fan is pretty low powered device and a typical dimmer might be able to handle the small inductive spikes it will produce.