An app could be reasonable if they build it into the United app (or whichever airline you're on) and get people into the habit of using it before there's even a threat of VDB/IDB. It could almost turn into a game that people play while waiting to board.

I'm imagining an app that's basically like having Priceline inside the United app. While you're waiting around to board, especially if there's weather or they've announced it's full, you can stand around and build alternate itineraries within your reservation and put bids on them. It should let you build at least three, and ideally should even let you include other airlines so that the airline can do the math in the background on what offer it wants to take.

Suppose I'm in a middle on UA for a transcon and the plane is going to be overfull. I don't really want to sit in that seat all the way across the country, and I see that there's a seat in first on AA that leaves two hours later. I'd consider *paying* them to pass me over to AA and take that seat in first that they can probably get for a discount, and I'd certainly put a low rebate bid.

After writing that, I think a good mobile-enabled website would be more effective as long as it's fast. People who fly once or twice a year are probably unlikely to have the app on their phone but might be entertained by building alternate itineraries anyway. You'd be able to build itineraries up to the point they close the door and tell you to shut off your electronics.

It would also wreak havoc on upgrades. People will come back here and post "I was #1 on the upgrade list and then got bought out for TOD by an VDB from a United flight. I think united is gaming it so that they deliberate oversell and upgrade people onto OALs." There would be another forum on FT on how to best plot and bid itineraries to get upgrades or maximum cash back.