[pedrocr]

>Nobody is talking about a gravity vector pointing towards the center of the earth during the entire maneuver. The implied assumption, I believe, is 1G towards the floor of the plane at all times.

I mean downwards towards the floor of the plane of course, not the center of the earth. mikeash has already explained much better than I could why you can't keep 1G from the point of view of the passenger and do a normal barrel roll from/to level flight. It seems easy to have a path that does it if you allow it to finish in descending flight.