stunt_muppet

Nope. Hence the lack of fic for Ben, Polly, Steven, Dodo, Vicki, Katarina, Sara Kingdom, etc. Of course, much of the fic being written right now is for New Who, so any Classic characters (with the possible-but-not-really exception of Sarah Jane) are rather rare.

isn't Jack/anyone automatically not that much of a rare pairing?

...Not really. See, it might be more plausible than some other rare pairings, but that doesn't mean people write it - he tends to get paired off with Nine, Ten, or Ianto (from Torchwood), and occasionally Rose or Martha. There's a nice Jack/Jamie over in the comments, actually.