Could there be an option to not write albumgain info to a file without an album tag? I ask because if I were to put all my music into the playlist and select 'Scan selection as multiple albums using album field', foobar will write albumgain info for all my single tracks, viewing them as a single album.

Isn't it easier to sort playlist by %album% and select for RG only files with non empty album field?

It's certainly no hassle but I don't know if its desired behavior to recognize a lack of an album field as an album.