Mikeymars-
I like your description of the immigration check, and yes, it is a response to the 9/11 attacks. We went on the 7-day in Sept.-Oct. 2000 and did not have to get up early for Immigration then. The cruise line was adamant in saying that this is required by the INS. It's only required of ships arriving in a US port from a foreign port (in this case Philipsburg, St. Maarten). If a cruise's itinerary goes from, say, Florida to San Juan to St. Thomas, I don't think they would have to do this.
On the excursion topic, I did hear second-hand that people that have early-morning excursions are allowed to go through Immigration early, instead of at the time specified for their deck, but I don't know for sure if they let anyone off the ship until the mass check is finished.
On a related note, there were uniformed security officers (I don't know if it was police officers or not) at the base of the pier in St. Maarten, checking for IDs and room keys from passengers from all 4 ships in port that day (December 2001). They wouldn't even let you on the pier if you weren't a passenger. This was in addition to the cruise line crews checking IDs at the ships. There was no such similar security at Havensight in St. Thomas, at least none that I could see.