This is a bit of a tough nut;
first a little clarification: Difficulty doesn't affect treasure pulls (nor does dying in the game), but Nightmare gets 2 completion tokens, Hardcore gets a Rare completion token, and Normal gets an Uncommon treasure token.
As far as not being overpowered/contributing to combats, currently the only way to know is to check the forum for groups getting together - usually groups on the forum are collecting for Hardcore or Nightmare, so the move would actually be to either find a normal group gathering, avoid the HC/NM groups, or post your own post. Unfortunately, a lot of the Normal-level or new players don't check the forum.
As far as fresh eyes for the puzzles, currently the best advice would be to get runs early on the first day of a con; though even that might not work after your first run. Also, at cons like South, West, and Gamehole Con, there will be more fresh eyes and fewer veterans playing on repeat. Still, it's no guarantee.
Now; as to whether something could be done to make this easier.
It's theoretically possible to adjust the TD events site (though I dunno about GenCon, which uses its own ticketing system) for players to self-report desired difficulty levels and past experience with adventures; or an automated system that indicates how many runs of a particular dungeon that IP had purchased. It would take some work and it wouldn't solve GenCon.
Now, maybe wherever tickets are sold could have a link to the forums, where we could make a greater effort to track/self-report which players have seen a dungeon before, or are looking for a group that is all new to the experience. That's the change that would be easiest to implement and has the potential for helping the most people, as far as I can see.
I came here to sing and collect tokens, and I'm alllll out of money.