I was flying DCA-ORD-MIA a few years ago on UA and the DCA-ORD flight was going to misconnect. Since I was travelling on a free MP ticket, I asked the agent if they could endorse my ticket over to AC to fly DCA-YYZ-MIA, since I had my passport with me. No problem.

Since the flights were on time, I had plenty of time to connect in YYZ and clear Canadian immigration/customs on arrival and then go through US immigration/customs a few minutes later. No questions asked - I just said there were problems with my flights and I needed to get to Miami that night, and the airline was able to rebook me on this itinerary.

My bags, however, "arrived" 10 days later. :-(

This got me reallllly curious, so I tried booking a IAD-YHZ-ORD itinerary on .bomb... and it prices out at USD $499 OW (base $440, tax $59, fare basis WOA21CNX). It *looks* like it's being priced out as two OW fares, IAD-YHZ and YHZ-ORD. So, since I have access to SABRE and it tried it in there, but not having availability to W class fares, I booked it in Q class (QOA21CNX) and scruitinized the rule displays... so what happens is that it's priced out as two fares, IAD-YHZ and YHZ-ORD. The itinerary shows "connection supressed" when I queried if it was a legal connection - never seen this response before. Anyway, looks like it's do-able from a paid fare perspective. Not sure what happens if you burn miles for a ticket on this routing...