I ran into this issue a few months back when a customer wanted dimmable under cabinet lighting. 406.3(F) says that if you start changing voltages, frequencies, etc. you need to use a unique plug so that other devices can’t be used, for obvious reasons. My initial thought was to use a twist-lock or some other NEMA configuration that no one would ever find on a household appliance. I wasn’t too keen on that idea and a bit more investigating brought me to 404.14(E) says that dimmers can only be use for permanently installed fixtures unless listed otherwise.
The answer is comes from Lutron, which makes a dimmer/receptacle combo. The receptacle looks like your ordinary 5-20R but it has a little “bump” between the flat blades, which prevents use by anything but their plugs.
They’re not cheap and it was more than this customer wanted to spend. Apparently her cousin had dimmed his lights for $20. Okay, what's next. http://www.lutron.com/pdfspecs/novatb.pdf