But I'm not sure if it's possible to do it with mobs without a coremod. I'll have a look when I can.
It makes sense that helmets would be transparent on mobs as well as on players, after all, it's the same helmet, it should look the same regardless of who's wearing it, right? =P
I too was horrified to see this. I had much higher thoughts of Gany.
I think Gany should adopt the Galacticraft's method to replace vanilla blocks/item with others of its own.
For example, the Galacticraft developer wanted vanilla torches to go out if placed in space dimensions. He then created another torch, with the same look as vanilla's but with no crafting recipe, that allowed the desired behavior. Whenever the player is in a space dimension and selects a vanilla torch in the inventory, the mod will then replace the vanilla torches with Galacticraft ones. If placed, the torch will look exactly like the vanilla counterpart except it will not emit any light or render flame particles.