I'm using a Sunbeam Snack Heroes Popcorn Maker as a base for my coffee roaster.
I've drilled holes in the back of the popper to allow me to hit it with a fan and slow down the heating. I've also added a hole in the side to insert a barbecue thermometer in the side to track bean temperature
I've ordered a variable fan speed controller from eBay which should allow me to control the fan/heating element to give me greater control over my temperature.
It is possible to rewire a popcorn maker to add a dimmer switch to the fan and heating elements separately in order to control both manually. At the present I'm not game to try it though.