Some SW employee commented on the SpiderMonster sneak-peak on YouTube, saying that Frank also came in to tape a second Mr. Johnson bit and a song. There's only one episode left of the season and I doubt those two segments would be in it together, so it could be one or the other or they both may not air until next season.
Was it said who Frank Oz performed in the song? It could be Grover, or it could be Bert or Cookie Monster, or it could be another one-shot Anything Muppet (I think it'd be a stetch to expect it to be Lefty or Harvey Kneeslapper or somebody like that).
While we're on the subject of Frank Oz, it seems like back when he did the Mad Men sketch, it was a big deal because it was the first time he performed a one-shot character in years, but then this season he did The Heaviest Catch, and it doesn't seem like it's as big a deal. Maybe it's because it's only his second original character in three years, or perhaps it's because season 42 isn't an anniversary season, or maybe the parody wasn't promoted as much (or it was but no advance announcements were made of Oz's involvement), or maybe the sketch just wasn't as good as Mad Men (that last one's my opinion, though). I would list the option of the show it's spoofing not being that great (I don't think I'd heard of that show before the parody, and have hardly heard anything about it since), but then again I've never watched Mad Men either.