It seems like I've been somewhere in O'Hare T3 and seen a bus to T5 that was, at the moment I saw it, primarily people connecting AA to a BA flight. The only reason I noticed at all is because they were doing a BP/passport check right there in T3 before you even got onto the bus. (Presumably then another check once at your gate.)

If the bus evaporates prior to your flight, wouldn't that be an argument that T3 to T5 should no longer be a 75 minute MCT? It would need to be longer, potentially enabling you to make a flight change for free.