What about this-
The IPac has 3 buttons that are shared for the leds. If you have a led and a button hooked upto the same input, and push the button, the led lights- regardless of num/cap/scr lock status.
I may be way off but you could hook up a buzzer and a light to that input, which would be activated when 1 of the players hits a button
THe obvious downside is that you couldnt make it "not buzz" if someone hits the button between questions for example. Also there's only 3 shared buttons, so no 4 player
Another thought- you could do this with any input, as long as you used a button that was rigged to a SPDT(?) switch- use one set of leads for the ipac input and the other to close a circuit on something like a doorbell