Addendum (09/20/18): Reader Jeremy Clowe has more insight onto Hall’s whereabouts that week:

I thought I would share this bit of info I found out last evening after listening to an interview with Rich Hall I found on iTunes- Amsterdam Comedy Podcast - #2. He talks some about the tough time getting used to the pace of SNL (similar to Martin Short I guess), and that the lowest point in his career came the night that Ringo Starr hosted. Like you write in you review, it's odd that he is absent during that December 8 show. What he reveals in this podcast episode, is that he spent all week working, had three really great sketches that Ringo loved and were going to be in the show, only to be dropped during dress rehearsal. He says they didn't go right during dress (too technical), so the producer cut them. Years later he interviewed Ringo and his All-Starr Band, and the former Beatle remembered and gave him a hug, saying that was a rough night. Mystery solved... but man, that sucks, would have loved to see Hall's signature style play off of old Ringo. It's like the "what if" of John Candy/Eugene Levy almost hosting.