I'm not sure that you can configure the Power button to do anything else when you stop the engine. It is a bit of a pain in the *** when you stop and have to switch the Radio/iPod back on but it's on one little push away.
As with Warren77, not sure that I'd run the anything with the engine off. You could only get the fan/blower to work as to power the AC compressor you need to engine to be running, I think???
. If it's that hot keep the engine running and if not then just open all the windows and sunroof.