Mike,
I recently flew to Japan, with my alembic and was able to carry it on. I had it in an alembic gig bag, (which is not small) and I just didn't take no for an answer. In evry plane their is space on- board somwhere, whether it is in the bulk head, a closest, or a flight addendant can find some place for it. Do be hostile, be polite, and if they still say no, again be polite. This is my experience. Most airlines are very accomodating to your needs, when you are pleasant. Also say you called ahead and they said it was OK. I guess you really could call ahead but I didn't want them to be warned ahead of time.
Best Wishes,
-Ed