I had a last minute extra guest for my trip last December. Like you, I called to change my ADRs only to find out I couldn't modifiy my reservation up by one on any of my choices. For three of the restaurants, I was able to add a reservation for one person as a separate reservation, figuring if all else failed, 2 of our party could sit at the smaller table. For the others, I figured I was going to hope for the best.
At all three of the places where I booked an extra reservation for a single person, I checked into the host/hostess podium and explained that we had 2 reservations and the party size for each but would prefer to be seated together. Each time, we were given a single table that seated everyone.
At the other restaurants, I explained at the podium that I had an additional guest and could they please add an extra person to our table. I was prepared if they said no we would eat CS somewhere, but each time, the host/hostess was able to seat all of us.