Inatalling the wiring and stuff might be a little difficult. But seeing as how it's nice and warm and insulated in there, I'd give it a shot. We have an old microwave too. I just might have to try it. Great idea!
I made my first incubator out of an old microwave. I took out the magnatron and all the guts and had a nice insulated box left. I used the stock cord and just added more wire where I needed it. Wired up a hot water heater thermostat and a computer fan. Used a small tupperware bowl and sponge for the humidity. There should be plenty of ventalation, mine had air holes at teh back and on the other side so I put the fan in front of the light to pull air over the light and mounted the thermo on the other side of hte microwave. There was actually a hollow void between the insulation where I put the light so its not in the main bator. I have enough room to put one egg turner that will hold 42 eggs.
I havent used it since I made my fridge bator, so I will clean it up and plug it in and take some pics so you can get an idea. It takes some time to get everything to fit right but once you get it you can be proud of it.
Please take some pictures of it. That sounds really cool. I had read about using the microwave as an incubator, but someone had said that the magnatron could fatally shock someone even when no electricity is running through it.