Starting with the Fifth Edition (1936), seven generations of the Merriam-Webster's Collegiate Dictionary have included (under the entry for tap) three definitions of "on tap," currently worded as follows:
on tap 1 : ready to be drawn from a large container(as a cask or keg) ["ale on tap"] 2 : broached or furnished with a tap 3 : on hand : available. ["services instantly on tap" —Hugh Dwan]
(The first two of these three definitions go as far back as the 1864 edition of Webster's American Dictionary of the English Language.)
But the Tenth Collegiate (1994) introduced a fourth definition for "on tap":
4 : coming up ["other matches on tap" —H. W. Wind]
The "H. W. Wind" cited by Webster's appears to be Herbert Warren Wind (1916-2005), an American sportswriter who wrote most often about golf. I have frequently heard "on tap" used in this relatively new fourth way in recent years, mostly in connection with impending sporting events. In fact, the San Francisco Giants radio network has a sponsored segment called "What's On Tap" ("brought to you by TAP Plastics") devoted to identifying and briefly discussing the next game or series of games on the Giants' schedule. I can sort of see how "on tap" in the sense of "in reserve" or "at the ready" might be extended to something like "ready to happen but still in the future"; however, the meaning "next on the schedule" seems another (and rather long) jump from there.
My questions are (1) did the fourth meaning of "on tap" emerge from one of the other three, and if so, how? (2) how far back in time does the usage of "on tap" to mean "coming up" go? (3) is this a U.S.-only usage, or do other forms of English use it, too?
I note that Eric Partridge lists two seemingly unrelated slang phrases from British English: "on the tap" ("begging for money") and "on tap" ("all modern conveniences, including h. and c.").