I did searches for quite a few names, and there were some that had no matches found. Names included Roosevelt Franklin (though he does appear in the background in a handful of skits included, and many character searches include skits where they only appear in the background), Bruno, Professor Hastings, and Don Music. I also did a search for "monotones" and didn't find anything, though the song Proud is included. But I found skits for Twiddlebugs, Two-Headed Monster, Biff and Sully (only one Sully sketch appears, and two other Biff ones are listed), Salesman (which also includes a few skits with other salesmen, besides Lefty, including ones where Grover is a salesman), Harvey Kneeslapper, and Herbert Birdsfoot.
I also did quite a few searches for individual letters and numbers, and it seems like there aren't any listings for skits involving individual letters or numbers.