In order to change the texture according to the durability, you need access to the models, because that's what defines which textures correspond to which item. That's why the vanilla tweaks one only does tools.
Myrto stopped his attack on the hound, instead going over to Davian and Branthin. He watched, holding his spear, frowning. Somehow, even though he had just fought, nothing about his disposition communicated that.
I could do it, if we had the textures. You could probably just cut one from an image (at least for the sword), or someone could make a custom design.
Don't quote me on this, but it might even be possible to include fractures in the shardplate when it gets damaged.
It's not really code, just a pack.mcmeta file and some model files that Blockbench can generate easily enough.
Myrto heaved the hound off of him completely, then struggled to retrieve his spear from its jaws, twisting it around and cutting the inside of the beast's mouth.
Myrto deftly stabbed the bloodhound with his spear, dropping it. He stood still then, watching the encounter, as though waiting for someone else to engage him.