I finally received a response from IB CS via Twitter. They unhelpfully told me that there is some back end glitch that prevents me from even seeing the option to select seats prior to check in. Of course, I already knew that! They were unable to assign seats and suggested that I do so myself at check-in.

I called the IB elite line and they were able to assign seats.

Looks like this is a known bug.

Thanks for the help!