Plex currently only shows these box sets as Disc 1, Disc 2, etc which is not very useful unless you know from memory what each disc contains or keep the artwork around.
There’s already an existing feature suggestion discussing an option to have labels/titles for individual discs (incl. the reference to MB’s discsubtitle). I suggest you comment/vote in that thread in order to avoid distracting or cannibalizing votes.
Unless I’ve missed some unique aspect of your suggestion, I’ll close this thread as a duplicate.