Chain lead shanks are preferred, unless things are radically different outside my home bubble. Part of the score on grooming and showmanship includes correct choice of and fitting of the chain. It can make life much easier when cuing as well. Much like a double bridle, I find that I can use a nearly invisible cue to square my horse up if he's wearing a chain vs. his plain lead rope.
As far as getting your boy more accepting of the chain, I suggest getting a short length from your local hardware store and attaching a snap to each end. Clip it into place under his chin on each side, adjusted tight enough that he feels it and it isn't dangling, but loose enough that it isn't hurting or rubbing. A couple of fingers inside should be fine... like curb-strap tension. Now you have the presence of the chain, but no dangling strap or changing tension on the chain. Just turn him out into a roundpen or other controlled environment and get used to the feeling, just like introducing a bit to a green baby. Slowly put the chain to more use as he gets the idea that it won't bite his nose off.
What the judge will notice is whether the chain is over or under the muzzle. Under is preferred, or at least so I was taught by my G&S coach a few years back. Over would tend to indicate a need of the chain to keep the horse under control, but is not unheard of. Studs commonly wear them over, and I see alot of the 'mad gamers' in 4-H required G&S use chains over. IMHO, if the chain over the nose is more comfortable for the horse, do it that way, or vice versa.
The only reason why I am considering this is because he has reared in a halter class without any pressure from the chain. Without any chain i can still get him to turn and set up without difficulty. Most of the time I never have to touch the lead shank he body off of body language.