Actually, looking at the ID3 specification, there is a frame intended for a subtitle for a disc within a set. The tag is ‘Set Subtitle’ (TSST).
So your set might look like this:
album (TALB) = A Song of Ice and Fire discnumber (TPOS) = 1 SetSubtitle (TSST) = Book 1: A Game of Thrones discnumber (TPOS) = 2 SetSubtitle (TSST) = Book 2: A Clash of Kings
Plex would have to implement support for TSST, so there is still a feature request here. Or maybe I should add it to the request for better ID3 support.