It took me nearly a full year...but I finally have an answer to the question I raised in this post. We're visiting our friends Tonia and Cason in Dallas, and as it happens, Cason is a major fan of Batman (or, as he might insist, "the Bat-man") and, when for some reason my dumb little theory about the musical access code in Batman Begins came up, he felt compelled to check it out. So, courtesy a bootleg DVD of the Batman TV series (incomprehensibly, still not out on DVD - maybe it'll come out on Blu-Ray...) and a perfectly legitimate copy of Batman Begins, I can provide the following comments:
1. I theorized that the little three-note sequence of clusters Bruce Wayne plays on the library piano, which allowed access to the Batcave via an entryway hidden behind a set of bookshelves, might have been the same three-note sequence of clusters often heard as brass blats during fight scenes in the 1960s Batman TV series. While I thought I had a clear memory of how that brass blat sounded, as it turned out, at least in the couple-few episodes of the show we skimmed, those brass blats had no particular scoring: they seemed merely to be groups of two-note clusters (at the interval of a major second) overlaid on the underlying fight-scene score when punches were thrown...but which notes seemed variable and irregular.
2. In the first instance of the piano key (har...), the notes Bruce plays are as follows (and I haven't found a good way to put musical notation in these posts): the C and D two octaves above middle C; the B and C a seventh above that; and the F and a fourth down from the second cluster. Amusingly, one musical figure abstractable from this is D-B-G...a sequence identical in intervals to the well-known NBC musical call-sign. (This would be significant except that the TV series ran on ABC.) Neither of us was patient enough to listen closely enough to the second instance (during the fire) of this piano business to note whether it's the same as the first one - except that I did notice that the notes are detuned in the second example (presumably to indicate fire damage to the piano), implying either that the access is mechanical rather than sound-based, or that I pay far too much attention to detail and I should really just relax.
3. Still: that the TV series used a series of two-note clusters based on simultaneous seconds, and that the movie used the same musical idea as a key, is similar enough to suggest that this just might not have been an accident. Finally: the second-based clusters are a key sonic element of the well-known theme to the TV series (the various interjections that sound like "Batman...").
* a new verb, coined based on the image here, and derived from the "Paul Is Dead" craze and the way fans desperately squeezed the most vague traces of significance from the most recondite, trivial, and incidental atoms in the Beatleverse in order to support a patently absurd notion...