Not possible, unless you modify the MP source code. Althou I cant see why it would be any harder to press back + stop. Both are using two key presses. Also if you press the green button on MCE remote control + stop you can MP to shut down automatically (as MCE green button opens the main screen.).
Not any harder I guess, its just that logic tells you that if you want to stop watching TV you'd press stop.. it doesn't occur to most people that you have to 'back out' of the fullscreen tv first - at least nobody thats picked up the remote for my HTPC anyway
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