I have the ARHA handbook from last year. It's almost identical to the AQHA handbook. I'm paraphrasing, but it says that apparel should be Western, but functional and super-bling is discouraged in both dress and tack. There is a huge emphasis on both the rider and the horse to look functional. Seems pretty laidback.
Just found it. Here is better paraphrasing:
traditional western attire that reflects a true ranch lifestyle
"Appropriate Western attire includes a long sleeve shirt with collar (buttons or snaps preferred), a western hat and boots with heels. ARHA's main focus is on the performance and ability of the horse and rider. Clothing with excessive crystals, glitter or sequins is strongly discouraged and in no circumstance will this have advantage over other exhibitors with well fitted western attire. The addition of any protective outerwear is acceptable as dictated by weather conditions."