I figured out what was going on, and thought I'd post it in case others have the same issue.

I found that the "Allow Realtracks substitution based on tempo" box on the "Song Settings" dialog was checked. I don't know when that option was added to BiaB, and I don't know if that was the default setting with the installation of BiaB 2012, but it definitely was not checked on my 2011.5 BiaB. With that option turned off (unchecked) I am now able to properly select any Realtrack. An interesting fact as indicated by the help flyover for the "Allow Realtracks substitution based on tempo" box is that the Realtrack display on the BiaB main screen is preceeded by the "~" character when the track being used has been chosen via substitution.