> (Sure, a flight near the Canadian or Mexican border could in theory divert to a Canadian or Mexican airport, but it's unlikely many people on those flights have their passports with them.)

There must be some procedures in place for this - I pretty regularly take domestic (Canadian) flights that pass over the US where I don't carry my passport.

A situation kind of like this happened to a relative in England; their international flight was diverted (I think due to weather) to a domestic-only airport; due to lack of immigration & customs facilities, everyone had to remain on the plane until it could take off again (presumably after being refueled) to get to an international airport