Teams buy their own tickets to avoid blackouts all the time. That is the way teams that never sell out get around the rules. A team won't do it if they are only 85-90% of capacity, but that 5% missing is only a few thousand seats for the Colts to buy and they still can resell them.
I still think the dome is not as big effect as it seems. There are several dome teams that are never really as loud. Considering Polian got into a physical altrication because he complained that the speakers were too close to the field and the Colts have been accused of piping crowd noise before, I bet Polian has first hand experience with speakers close to the field and piping in crowd noise into the RCA dome.