Thanks, all. Integration issues seems like the likely cause. The flight is booked into SAS Business Smart (D class), and DL states that we are confirmed on the BOS-IND flight in domestic first with no apparent ticketing issues. For whatever reason, it seems like advanced seat selection is just not possible. Flight is SK coded.

UPDATE: I just called again. They were able to assign seats today (T-5 days before departure)...of course once the Platinums took the good seats! I assume they have additional control now that departure is approaching.