With all that said the fact that they have the campaign character's having different HUDs means that they have at least laid the ground work for allowing something like this in Multiplayer, I mean they could reuse the HUD elements they built for campaign and then make more for the various other helmets that you can choose from and then just make a variable that stores which assets to load for the HUD based on the name of the helmet chosen. Yes it would be more work but I think it would be very cool and add more depth to armor choice than just pure aesthetics.
I don't think it would be unfair if they did something like this any more than picking what gun your loadout starts with it would just allow you to pick a HUD that works more with your preferred play style just like you can pick the gun that works with your preferred play style within the given options.