I've been looking around and judging by the answers everywhere this can't be done?

But I would really need it, is there some existing module from marketplace that has the code for this (custom widget or something)? Or is there something that would visually look similar?

Basically I have a custom model (custom.model.line) with this hierarchy: product_category -> product_template -> product_variant and I need to show it inside the (custom.model) form (not a separate tree view) as it was requested by client. All I need to do is to somehow visually represent the hierarchy.

Currently I managed to get it somehow working by playing around with board.board but I have to fix problem after problem (currently biggest problem is domain not recognizing field for action).

Another approach I have in mind is to use section_and_note_one2many widget and then for sections I can use product_category and for notes I could use product_template. However doing this I would have to program the hierarchy (line positions) myself. Since this document I'm doing is the base for everything I can't mess it up and would like to keep it as simple as I can.

Any ideas?