Two obvious problems with this proposal: IAH and UA

IAH has international departures from many terminals, they'd have to give up a lot of gate scheduling efficiency to do segregation this requires. Or build a second level on the terminal like FRA does with A/Z.

Given UA's poor cancellation rate, they're particularly ill-suited to be the airline that does this. DL would be more viable if congress and CBP were open to it, because they'd strand fewer passengers in the process.