Hats may have been marked feature complete on the release of 3.0.0, but with the release I’ve included a method for Hats to get mappings for custom mobs (like Morph’s Abilities).
At time of writing only Thaumcraft’s Pechs and Golems are partially supported (with thanks to Lomeli12). If you’re interested and you’d like to add mappings of your own/tweak existing mappings, the mappings (JSON file) can be found here, with instructions on how to create mappings here.
If you get a hat on a mob looking right, feel free to submit a Pull Request (see instructions).
I’m hoping this gets some of the community’s attention like how Morph’s Ability mappings has gotten, so please spread the word!