I found an inconsistency in the way foobar2000 creates the subsong index (%subsong%). In cue sheets, the subsong index starts with '1' for the first track. In MPEG4 and Ogg files with chapters, the first subsong index starts with '0'. Files that have only a single track in them MP3, Ogg, MPEG4, (didn't try this with cue sheets) all have the subsong index starting with '0'. It seems that cue sheets are inconsistent with the other types of multi-item files.
But interestingly, it would be useful if all multi-item files would start with subsong index '1' and files that are only single items (MP3s, Ogg and MPEG4 files without cue sheets / chapters) should have their subsong index = 0. It would make it simple to check if a song is part of a multi-item file within the playlist by checking if the subsong index is greater than 0.

Anish

How do you use this tag with MP3s? I asked on IRC and was told it wasn't possible.

I don't actually generate the tag myself. It is created automatically for multi-item files. Since in MP3 doesn't support chapters, you have to create a cuesheet to create a multi-item file.
Anish