I gather that it's quite a bit more than a "bit of a no-no." Wife and I were stuck onboard a sweltering 739 w/ MX down in LIR awhile back. They finally sent us back to the terminal since no one on the ground could identify any sort of technology that cool a cabin. No sooner than I paid like $25 for a couple of minis and a few mixers they sent us back. No prob - we'll just have a little pre-departure cocktail. I have never been read the riot act by an FA quite like that. One or two more stopped by to add their admonishment as the flight progressed. I apologized profusely and they eventually cooled down, even joking about it by the time I was stretching my legs near the galley by the end of the flight. Got the message loud and clear, though.