So in the hunter rules, I have found references to "regulation snaffles" but I can't seem to locate the section where legal bits are actually shown and described. I've looked through the hunter rules, the general rules, and a few other pdf files that are part of the rule book.
I've only shown my (not-so-baby) baby horse at local shows thus far, but he shows in the hunters in a loose ring, because all of the double jointed D ring bits have mouthpieces that are too think. I've considered getting a Sprenger KK D ring that seems to be about the same thickness as my current loose ring (a KK knock-off) but not sure how he would like the fixed cheek.
As far as I'm concerned, if a judge doesn't place me because he's in a loose ring, it is either as a minor tie breaker, which kind of sucks but whatever, or the judge has a questionable judging criteria, which will become evident in other placings and I will avoid.
No legal/illegal in the hunter ring. It's conventional vs unconventional. The only things set in stone (off the top of my head) are boots on the horse or a martingale in the hack. You could technically show in a hack a bit or a figure 8 with a running martingale with a gag. Will you get a ribbon? Doubtful. Your best bet is a snaffle, something that can pass as a snaffle or a Pelham.