Very cool, but sadly not too practical (very vulnerable to shoulder surf attacks, and it's not that hard to tell that there's something funny about the knock).
I think a sufficiently long knock (just like a sufficiently long password) would still be almost impossible to crack. If you go on for 15 seconds, who's going to be able to get it right? Not that this makes it practical. I'm just saying. It's only if the knock is really short that this is a problem. If you knock out 30 seconds of a song (assuming it's obscure enough that no one will be able to determine the song), there's no way anyone who doesn't know what song it is will be able to reproduce the knocks.
I think it's better suited for rooms inside the house/apartment rather than the main entrance, for the reasons you stated. This would kick a%$ in dorm rooms.
