Darn you RV.net. I just ordered all the parts to do this to my old crappy tiny bathroom fan. Heng retrofit, PWM, AND two gang wall switch! Mine is currently wired with the light on one switch, and it's all but impossible to reach that tiny button on the exhaust fan when you are on the throne. This way you can turn it off/on at your leisure, but then use the PWM dial to preset.
dmullen - if you are still using the one with the fixed dial, assuming you have enough vertical clearance I'm not clear on your question. Won't the circuit board just be sticking up in the air at 90 degrees to the housing?
Either way you could use double sided tape. It shouldn't hurt the circuit board. The moisture would be a long term damage issue, but you'd need a lot/constant very high humidity. The whole point of the fan is to move the air so I don't think it's a problem.