Interesting that you discuss Seattle. It was at SEA earlier this year that I and a friend boarded an American (either 757 or MD-80, I don't remember which)after volunteering to be bumped. They said the plane was only oversold by two so they probably wouldn't need us but thanks for making the offer. Just before the aircraft was buttoned up, the gate agent came and found us, asked if we still were interested in volunteering. When we said yes, she said something like "send them down" on a walkie-talkie, told us to get our carryons, pulled us off the plane (and I literally did see the door swing shut before we left the jetbridge) and gave us $500 bump vouchers, a hotel room, and first class seats all the way to the east coast the next day.

So I've had bumps happen both ways, but being asked to wait outside is more common and I think for the reason that's easier for the ground crew. Maybe a good in-between solution would be for the gate crew to let the cabin crew know these last-minute boarders are unneeded bump volunteers and so a little extra help in finding space in the overheads would be appropriate?