That is a TheTVDB issue. Since Plex uses their DB to indicate the season order, and they do not allow specials to mixed in with a particular season. If TheTVDB allowed things like episode 4.5 to indicate a special that came between 4 and 5, Plex could be adapted to take advantage of that.
That is the purpose of Aired Before though, right? For example, if the Aired Before is set as s4e10,then the special is essentially s4e9.5. The data is all there on thetvdb, it just has to be used by Plex.
