A training fork could be worse, because they act on the bit (if I'm reading this right - it's the same thing as rings). Also, you can't use rings with a curb bit. But if you did try rings, get ones that have their own girth strap as well as a neck strap, because a martingale attached to your breastplate is only going to interfere with the breastplate's work.
Oh, is your tiedown elastic or solid? If you're worried about interfering with him, an elastic tiedown would give him a little more give, as well as some warning before the full strength of it kicks in.
Also, the mare that braces, she doesn't brace all the time, just keeps tension on the tiedown when she turns to help her balance because it's the way she was trained. If they're constantly fighting the tiedown OR rings, they'll end up with uber muscles under their neck and no topline.
Just a thought, so don't yell at me, but you could try running reins if they're legal. That way, you've got full adjustment capabilities - give him space when he wants it and stop him from being a dork with his nose in the air. Of course, they're also only meant for use with snaffles, not curbs.