I've been thinking about this lately as well. I believe I even mentioned it somewhere on the forum this past week.
I don't think Forgetful Jones being originated by Michael Earl rather than Richard Hunt would necessarily mean they weren't available. When interviewed for Tough Pigs, Michael Earl said that they had a meeting to figure out characters for him to perform, and he informed them that he could do a cowboy voice.
Michael Earl has said in season 10, his first year as Snuffleupagus while Jerry still provided that voice, that Jerry did the voice live on the set, but when Jerry Nelson was interviewed for Tough Pigs, he said that somebody suggested he stop performing the character because he was busy with The Muppet Show and Snuffy was primarily a street character. With that in mind, it would have been hard to do the voice live. I wonder if facts were mixed up, like maybe he did the voice live on set when he was available but also pre-recorded the voice.
This season also featured reporter Warren Wolf, who seemed to be a substitute reporter for Kermit.
And of course, none of the new characters from season 12 were performed by Jim, Frank, Jerry, or Richard. They were all performed by Michael Earl, Brian Meehl, Karen Prell, and even Kevin Clash, a few years before he was a full-time performer with the Muppets. I wish I knew more about the circumstances allowing him to perform in season 12 was, particularly the fact that he had a few regular characters. This fact is rarely acknowledged in any official publications from Sesame Workshop (Kevin Clash did mention visiting the set and lending a hand when he could before season 16 in his book, but he seemed to be kind of vague about it).
But there are segments with Jim, Frank, Jerry, and Richard that do have official sources saying they are from season 12 (not sure if any of those sources are incorrect). I believe Hulu lists the Sesame Street News segment involving the first day of school as a season 12 segment. Old School Vol. 3 lists the "beginning, middle, and end" segment with Cookie Monster and Kermit as from this season. If I Were is copyrighted 1981, which could be from season 12 or 13 (the copyright dates for most Sesame Street songs are the year that each season ended at). I think Fuzzy and Blue is also from this season (I've seen it with a 1981 copyright, can't remember if it's on Hulu or not).
And on a side note, it seems like Northern Calloway wasn't in many episodes this season. His name was removed from the credits, most likely due to the personal issues he was facing in real life at the time, so it seems like he was a part time character this season.
I was just watching the street scenes from a season 12 episode, which had the end credits included, and I saw Northern Calloway credited.
Slightly off-topic, but back in 2009, when I saw part of a season 12 press release talking about the new characters being introduced that season (though not all of them were introduced that season), I figured they'd all be fairly major and introduced at the beginning of the season. Now that the first few weeks have surfaced, it seems like they actually took their time with several of these. There's a few that do not yet have full pages on Muppet Wiki, and the copy for one episode has some sort of error preventing it from including the full episode, but I am amazed by how some characters appear sporadically, and a few still haven't shown up in any of the episodes in the guides (but as I said, there's still a few left).
Telly is the most-used of these characters. While Forgetful Jones, Clementine, and Buster were all in the first episode of the season, Forgetful and Clementine didn't return until later in the second week. Countess Dahling von Dahling and Masha both appeared twice in the first week, but so far the Wiki doesn't note any other appearances by them (though one of these episodes has The Count with a Countess, who I assume is a different Countess, though both were performed by Brian Meehl).
Of all the recently-surfaced season 12 episodes, the only material with Elmo seems to come from the previous season (that press release referred to Elmo as a character who talks in sounds instead of words, it'd be great to see a segment where this applies to the character). Poco Loco appeared a fair bit in the first two weeks, it seems like he had four scenes. While Deena was in the background in the second episode of the season, a Deena and Pearl sketch did not appear until late in the second week (but after that we've found a few street scenes with Deena, still no return of Pearl). And looking at these guides in order, it wasn't until tonight that I found season 12 episodes with Dr. Nobel Price and The Honkers.
Of all the characters promoted as new characters for the season, the only ones who have not yet shown up in the season 12 guides are Warren Wolf and Ferlinghetti Donnizetti (the latter being the only character mentioned who was not in the group picture of new season 12 characters).
I don't know how far production of that season came when the press release was made. Maybe the press release came after the first several episodes were edited and completed (and maybe Ferlinghetti Donnizetti was created after the group photo was taken). Maybe they didn't think they needed to introduce all of these characters at once (looking at the first two or so weeks of episode guides, it also took awhile for Michael Earl's Snuffy to show up, while most of the other classic main characters showed up, in classic material, quite a bit in those weeks). Maybe with the large amount of new characters they felt they needed to not introduce them all in the first week (especially with a large number of classic characters).