I think we're overlooking the simplest solution. If they can't get the plane out for whatever reason then I think it's perfectly fair to have the airline endorse the ticket.

That gives the PAX the option to see if UA or AA would accept the remaining coupon for travel. Those of you who remember NW's Bring Us More Passengers (BUMP) program will recall they were quite happy to intercept revenue form other airlines, even if the coupon was below the walk-up price.

Worst case, a competing airline wouldn't take the ticket and the PAX would fly the rebooked routing.