you should consider adding support for creating icons with custom tags in game, considering the mod can display everything i threw at it so far. for example:
custom heads

enchantment glint

right now you have to manually edit the menu.json, and with a simple python script i can pull and format the data from a /give command generated with an online tool, making it a relatively easy process.
with that being said, i don't know how you'd be able to do it, but i think you're smarter than me so chances are you could figure it out.
not having access to custom heads feels restricted, so really give it a thought, with like the 1.21 update or something

tested on 1.20.4 fabric

i figured i'd share the script for anyone who wants mess around with custom heads and stuff
script.py
just paste the give command to standard input and it will poop out the proper "{Count:1...}" shebang. you can pipe it to wl-copy on wayland or something and paste it in the menu.json
it might not be pretty or neatly organized but it does convert all edge cases i could think of ¯\_(ツ)_/¯

Added now, with the same format as it would be in the json :)
Had to change functionality a bit for the icon pick screen to achieve this, but nothing too major.