Unfortunately, you can not change the block model of armors easily through a texture pack, which means when you're making a texture pack, there's a pretty big limitation of what you can do.
I'd like a mod that lets you add textures on a custom block model. Being able to customize it like you can with blocks would be nice, but even if there was a standard one that gave you more options than a cube, it would be great. Just extra planes to draw on so you can add cool features.
I think it would work if you just replaced the helmets box model with something like this it would work:
The side pieces outside of the cube would just be to add hanging things on them, like the side of cacti. You'd leave it transparent if you didn't want anything there. The textures would have to have a different layout than the normal ones.
This mod should only replace the model of an armor piece when a special texture is found in the config folder. So you wouldn't have to make new armor textures unless you want to. It should also let you replace the armor of mods, such as metallurgy.
If you think this is possible or like the idea, let me know.