I was on an AA BOS-MCO flight in January that was delayed seven hours. (Mechanical problems; they had to fly a part in from New York.) The aircraft stayed at the gate. We were allowed on and off at will with nobody checking. I don't recall the cabin crew counting noses before we finally left; they might have, but if they did it was pretty subtle. Several passengers did leave: some canceled their trips, some were rerouted on AA (especially those who were connecting through MCO to somewhere else), some flew on another airline. I doubt AA had an accurate count of any except those who were rerouted on other AA flights.

I don't know what happened to people who boarded this flight and then decided to bail. They may have gotten the miles for it. (My travel companion and I each got 9,000 goodwill miles from AA, too.)

However, it's hard to plan ahead of time for this sort of thing, so even if it worked in this case it wouldn't make much of a strategy.