If he is good at dressage for H/J horses and riders, I bet he is used to the H/J attire and will not clutch his pearls when he sees your barn roll out in their customary H/J outfits. Dress the way you would for a GM clinic.
For a dressage clinic, I typically see the horses are bathed and clipped, with braided manes at the Very Fancy clinics and pulled/trimmed manes at the other clinics (FTR, I have never braided for a clinic with a BNT). Clean saddle pad, usually white or a non-distracting solid color, and some pairs go for matching polos. Personally, I like to keep the horse’s legs bare as I think polos can be distracting if they are white, though they do look good… Tail should be free of debris and combed out, we bang tails before a clinic but I think in H/J world you have a different way of dealing with tails.
As far as saddles go, ride in the saddle that fits your horse. It should not matter - you are not a dressage rider, so why would you ride in a dressage saddle?
As far as riding prep… work a lot on your transitions. Many of the clinics I have attended have really honed in on the accuracy and timing of good upward/downward transitions. Polish your leg yield and half-pass (if the horse knows it) and have fun!