Hugh,
I hear your frustration.
The BIAB SGU/MGU format that Realband uses is a two way street that allows transfer of files back and forth between BIAB and Realband. For this reason, this file format only relates to the blue BIAB tracks found at the top of the Realband tracks (since BIAB only has 7 instrumental tracks). Any normal Realband tracks are not saved in the SGU format.
If you open "Preferences" and go to the "File" tab, there's an option at the bottom of the settings window to set SEQ as the default format.
Regards,
Noel