My local chapter is offering western dressage at all of our schooling shows (which are not recognized or sanctioned by our GMO, but run under USEF/USDF rules). There is quite a heated debate regarding the requirement of helmets. The county facility where the shows are held does NOT require helmets (even for juniors), but a release must be signed. When consulted, our GMO suggested requiring the wearing of helmets. The potential WD competitors, most of who are cross overs from WP and reining) are understandably resistant to the requirement (as a side not it is NOT required by USEF for reiners to wear helmets). So what do the shows in your area do regarding the helmet rule?
local multi discipline schooling shows here require approved helmets for all juniors, adults don’t have to wear a helmet western (but it is recommended), but is required apparel for english - I think they allow non approved helemts for adults in the english ring as it is a dress code not a safety requirement. Waivers are required either way.
If you are not a sanctioned show I would think the insurance company’s requirements would be the final say.