I've been wanting something like this too. It seems like I get near the bottom of a keg, and then attempt to kill it, thinking there's only a couple pints left. So I'll be hitting a keg like crazy, and that thing just seems to keep on pumping out the pints! Honestly, I think the best way to make a keg last is to try to kill it.
So I've resorted to bottling from the keg when I want to put another one in rotation. Just bottle the rest of the keg to clear up space. So once or twice I've gotten all set up for that process, start to fill a bottle, and poof. There it goes.
I was thinking there should be some sort of buoyant magnetic wheel that could roll down the inside wall of the keg, and a weaker magnet on the outside wall that would follow along with the wheel. It would be pretty tricky to balance out the magnetism correctly so that both parts sink rather than hanging high. Hmmm...