This would be a nice feature. All heroes look the same in the end.
I think it seems odd to want the bonus of an item but not look like you are wearing it. Just don't put the helmet on.
However,because all champions are all magic wielding masters of the sword they are all the same. I appreciate the customization but because all champions have access to magic and are master warriors they all act and look the same eventually.
They need to change how champions develop to eliminate this. Make limitations based on what the champion is doing. Example, a person trying to shoot a bow and arrow sitting on a horse in full plate mail armor---would never happen. If they could pull the bow back, which I doubt, they would not be able to hit a damn thing. Full chain mail would be questionable, but at least probable. Leather would be the most practical.
There needs to weapon armor combinations that do not work together. In my games, anybody that is casting magic, can only wear robes or leather---leather is rare, but I make the exception sometimes. Never Chain or Plate. Also any "magic-user" cannot be armed with weapons other than a staff of some sort, or sometimes they can have a dagger.