Modify Item.Properties for item form of block auto-generated by an element

Lets say I have a block element called TestBlock. And now, lets say I want to add the minecraft:equippable data component to TestBlock's item form to let me equip TestBlock on Llamas, like you can with carpets.

Setting it to not have an item form and registering said item myself in ModItems (where I can successfully add my own Item.Properties) causes the item's model file to always be obliterated whenever the block is regenerated.
Is there any way at all to do this? So far every method I've thought of (short of just making the entire block from custom code as you would in a normal modding environment) has major issues due to Mcreator's very insistent code regeneration.