Theoretically it CAN be done with some limitations if the oven's big enough.
Lots of new car manufacturers now are e-coating bodies (similar to powder coating but different) and there are vast technological advances in the industry all the time. As callofthemopar mentioned, wood and mdf can be coated nowadays, and there's even a portable method for swimming pool liners that cures in the sun! To date though, you still need to be able to invoke a charge to your metal for the powder to adhere evenly, and it will definitely not stick to Bondo.
I've had pretty good luck with interior panels over the years but haven't had a chance to try a fender or quarter panel. The interior stuff holds up fine to the curing heat with no warping or adverse affects. (Don't try it with anything that's ever had Armor All or other silicone based lubricants on it though -- the results will be streaky and uneven, and no amount of prep or outgassing will ever remove Armor All from the metal.)
There are several high temp metal fillers on the market specifically designed for powder coating -- ThermoBond3, Lab Metal, et al. -- but in my opinion they ALL suck. The only thing I've ever had luck with is
a product designed for exhaust repairs made by Permatex. It's easy to work with, sandable, grindable, doesn't shrink much, sets up fairly quickly, and a little bit goes a long way. When it's done right, repairs under my powder work are invisible. The only drawback is that it's $12.99 for a .7 ounce blister tube.
You'll also need to remove EVERYTHING from the body that will not withstand 500 degrees:
every piece of rubber, every seal, all the weatherstripping, every piece of plastic, etc., etc., etc., has to be pulled off, and you'll need some kind of sturdy support jig that won't touch or interfere with your powder work with metal wheels so you can roll it into the oven after the coating is on there.
Keep me posted -- I wanna see it when you're done.
