I did a bit more digging on the origins of the click track, which has apparently been used since the 30's In the film industry, not so much for time keeping as for synchronizing music to image more easily.
"Lou Fleischer did it first, no? The "click" and the little pin prick holes on the film where the musicians could follow the sync. The click was only heard in the cans of Lou (the conductor). The pin pricks on the screen "bounced" like the famous Fleischer Bouncing Ball so the musicians could anticipate the beats. This was 1932..."
(http://forums.stevehoffman.tv/threads/origin-of-the-click-track.157756/)
although Wiki credits Carl Stalling (Wassup doc?)
Concerning Classic Rock, here is a response to a thread I found over at gearsluts:
One of the best click tracks of all time = Money, by Pink Floyd
"I believe it was Mr. Waters, with maybe some help from Alan Parsons. They used 7 different sounds, all related to money (we all know these sounds, right?), cut each sound to tape. Spliced each sound, back to back, and using a broom handle to keep it taught, they ran it through a tape machine over and over."
Apparently you can here a click bleed through the vocals of the Monkees "Daydream Believer" too.
It also seems that Bob Ezrin put an SM-57 in a box and tapped on it for a click track on the sessions of Kiss Destroyer.
Sly and the Family stone used an early drum machine on "Family Affair"
I also read that Keith Moon was doing this as early as 1971, but I have found nothing to back that, so Who knows.....