We did this (no checked bags) on an itinerary ORD-IAD-LHR.

Asked at the customer service desk at ORD if we could get on ORD-LHR, only to be told no.

Went to the gate and talked to the GA for ORD-LHR and got on the flight.

GAs have a LOT of power. May have helped that I was 1K at the time, but worth asking.