I'll update this for the record. As the date got closer, I was able to improve my seat assignment, and took my carry-on to the gate. They announced before boarding that the flight would be full, there could be a problem with the overheads, and pax were invited to have their bags tagged for gate check to the final destination. I got that done at no charge, and we were invited to preboard to take the bags down the jetway.

The first flight was delayed, adding to a tight connection at ATL between high B and high A gates. I got to the second flight late in the process, and probably would have had a problem finding overhead space.

So, I wound up checking the bag for free, had easy boarding on the first flight and, since I was waiting for people on later flights, the wait at baggage claim wasn't a drawback.