The tone detection system seems to be the easiest. I'd probably even do the decoding in software using a fast enough uC+ADC. For instance, DTMF decoding can be done in software with a modern microcontroller.
As for receiving simultaneous signals, I am content with just flipping a coin if the signals arrive within a certain amount of time (like 1/100 sec.) Besides, aren't the idiosyncrasies of the buzz-in system part of any game show?
Just wondering if there is any other ideas that I've overlooked. I wouldn't mind doing some form of time-division multiplexing if there was a simple way to synchronize the buzzers with the base station.