I'd put money on the upcoming august Crew-9 flight of a Crew Dragon to become the rescue craft. It's scheduled to have four crew fly to the ISS for a six-month mission, but I think NASA would probably fly it up with just the commander and pilot, and let the two "stranded" astronauts fill the roles of the two mission specialists for the full mission length. This would also give NASA, Boeing, and SpaceX time to figure out a way to connect Starliner pressure suits to Crew Dragon power/data/life support systems, and fly up that adapter hardware on a cargo flight.

The Crew Dragon is theoretically capable of flying seven crew, but that seat configuration has never actually been used on any orbital flights, and there's no way in hell NASA authorizes a major modification to a crew vehicle just weeks before it's due to launch.

Interestingly enough, if the two Crew-9 mission specialists are bumped off the flight, there are geopolitical-tension consequences. One of the mission specialists is Russian. NASA and Roscosmos have miraculously maintained a cordial working relationship despite waves hands around in general.