I came into this thread with a preconceived notion that it seemed like a bad idea (forcing people to party on a given set of jobs is just asking for outcry from people who don't have friends/want to level job A or B). Having read your post, however, I believe the idea has some merit - with a few caveats.

This cannot (and I can't stress this enough) be a large part of the game. As a little bonus dungeon, though, it's an interesting idea. Something that allows players to really test their mettle on a given job or set of jobs when compared to other players. I would make one addendum however. I believe that instead of restricting players to Jobs A, B, and C. The "dungeon" should have a whole set of pre-designed scenarios that are chosen from based on the party. In that way, instead of it being "you need jobs A, B, and C" it becomes "if you have jobs A, B, and C, you do this mission". That would put more freedom in the hands of the players.

As a personal note, I would think these should be as hard as possible, given that the entire point of the idea is to really test the players' skills in a controlled environment.