Title says it all, just want the ability to have "custom items" for use as icons using custom_model_data predicate.

You can use NBT data, it's undocumented at the moment:

Added support for NBT tags in icons. You can add NBT after the icon's material.
Example: ICON: diamond {... NBT data here ...}

I don't know how much out of scope this would be, but maybe also have a command that would return what NBT data the plugin would see for the item you use?

@Andre601 do you mean, for example, an item in your hand?

@Andre601 do you mean, for example, an item in your hand?

Yes, exactly.