I think I've just found a new security device for my front door- I'll just be looking for a remote control microwave oven now ;-)
Just hook the microwave up to a pressure switch under the door mat, or for a remote use one from a ceiling fan. Ultimately, add a wireless camera as well so you know for sure who you're cooking.
If you want to make the unit much smaller, just take the MW antenna and its' circuits out of the case, and remove the door interlock switch by jumpering it out. Then the whole unit is smaller than a loaf of bread.