Stretching - Heating and hammering hot. This is something I have done only forging solid pieces.
Shrinking - Heating and Rapidly cooling, hammering cold. Something I have yet to try on a panel. I heard it works well. Just have to get up the nerve.
I have seen people do this, you can get good results with heat, however keep in mind that when you heat up this type of metal and hammer on it, you will make it weaker. However much like you stated, seeing that these panels are not easy to come by and are not cheap.
Goody, you can't hurt anything by trying to pull out the dent, drilling it (then add metal back), hammering it, or heating it. Use what ever means to make it look usuable again. If you can't get it completely flat or straight a little filler never hurts. If you don't have the tools or time, take the fender to a shop that can make it straight again.
Just remember, something is better than nothing.
Just my 2