There's a 1.8 version for this mod, however I can't get it to work with 1.8.9, even if I modify mcmod.info to directly target 1.8.9.

Is there a chance for an official 1.8.9 version?

Yes, there is a chance, but other parts of the mod need to be updated for later versions first.

Okay. Also, is there a way for resource packs to customize the armor bar?

@LewisTehMinerz replace this texture
/assets/armorchroma/textures/gui/armor_icons.png

Got it.