What kind of show are you going to? I don't think that there would be any problems at an open show, but isn't the arab assoc. A little funny about mane braiding? Or is it just purebreds? I'm assuming Onyx is a purebred, he sure is a nice looking guy.
Try some different things when it gets closer to show time. Use different strings and yarn, try hair spray and gels, My aunt sprayed her hair with Aqua Net back in the 60's and it hasn't moved yet.
We have a few open shows this month and the 4h show, I don't really care about how he looks for those shows but next month is the Spokane Sport Horse show, the entry's fee's aren't cheep so and it's a big show for this area so I'd like him to look his best, also we are planing on taking him to the Sport Pony Inspection in September so I want him looking his best for that, Onyx is a purebred but his last registered owner isn't willing to give up his paper's, but since he is eligible to be registered as a sport pony were going to do that rather than trying to fight with dumb people trying to get the papers...