Jock,
That nursery rhyme illustration doesn't help us on this side of the Pond. Some folks say "baa, baa" with an "ah" vowel, as used in "block" or "walk". Others use a short "a", as in "apple" or "black".

Is the -aber portion of Lochaber equivalent to the -aber in Abercrombie or Aberstwyth? If so, I've heard it used with the short "a"