Maybe it was this post by me that he was referring to?
http://forum.dominionstrategy.com/index.php?topic=1431.msg23933#msg23933
I've tried to find synergistic cards with Jack of all Trades on the http://councilroom.com/supply_win page. Maybe I should work on making the data easier to copy to the forum? But JoaT seems pretty damn resistant to helping anything.
Maybe Inn is the card that gains the most? +3% chance of buying, win stats basically the same, an extra .07 inns bought per game.
Maybe JoaT tends to shorten games, most cards are purchased less often when its available.
But you're looking for something else there. You're trying to see if the presence of Jack makes certain cards more likely to be part of a winning strategy. The comment was about X+Jack+money being better than Jack+money. This kind of question is better answered by simulation, and the simulator says the following $5 cards (possibly among others -- all I did to make this list was take the default bots and add a jack or 2 and compare to the jack bot) X satisfy this:
bazaar, cartographer, council room, duke, embassy, explorer, festival, inn, lab, library, merchant ship, mountebank, rabble, royal seal, stables, stash, torturer, vault, venture, wharf, witch
That's a pretty long list, which suggests that Jack+money is a pretty poor strategy. This means that even in the presence of jack, you have some interesting decisions to make. If there are more than one of these cards around, which one (or two) do you go for? And this is just $5 cards. The lower cost cards may also influence the decision between the 5s just as in a game without jack.