I have a block, with a tile entity, the block uses of a BakedModel to render based on some data from the tile entity. I've made a custom BlockItem which gets dropped with the tile data serialized to a nbt tag, and that when placed, deserializes the nbt for the tile entity.

The baked model is simple. I have a base model defined in json, and the getQuads simply adds a new cuboid shape inside the base model, representing how much the block is filled (The block is a jar)

but the Item renders just the base model (as would be expected). How could I make it render the model from the BakedModel? I've looked into ISTERs, but it doesn't seem that I can get the BakedModel quads from there

If you already have a IBakedModel implementation, you could create your own ItemOverrideList that gives you the correct model based on the stack tag.