Around 1993-1995 the late Phil Hartman hosted a special which aired on FOX in which he showed what it was like working on the Simpsons. They showed the animation process,story boarding,voice recording etc. There was also interviews with the cast and others who worked on the show. At the end he even met The Simpsons themselves. I have never seen this mentioned anywhere and there are no clips online.
This is something you remember watching? Are you sure it's not a faulty memory? That is weird it wouldn't at least be listed on imdb.
I remember a few details about it. It opened with Phil walking down a hallway and he said something along the lines of. "Hi. I'm Phil Hartman. You may know me from...hosting this special" I also remember at the end he met the Simpsons themselves. They were animated and he was live action. I did see somebody mention it somewhere else because they were wondering if it would be included on one of the dvd sets. May have been on the no homers club forums. But that was years ago.