This has been bothering me, kevs, and I think I just hit on why.
Depending on how you've got your songs sorted, i.e. which iTunes column is named in bold print, newly added songs may, but only may, go to the bottom of your 15,000 song list; a "Name" sort will group all songs with the same name together, which, if I follow your OP, will allow you do what you want to do, easily.
I'll concede, though, that I may have misunderstood your OP, because playlists seem to be both involved and out of context at the same time.
