Well, safely back in the USA. Figured I'd report on my experience. You DO have to go through passport control and customs. Thankfully, my upgrade came through, so I was one of the first off of the plane. There was no line at passport control, and about a 2 minute line at customs. I was told I did have to pay the departure tax, and obtain a paper boarding pass from check-in -- both of which took longer than customs.
Of course, the Liberia airport is very small, so things were close-by. I had plenty of time to kill with 2 hours in between flights. Of course, YMMV.