Filter parent records within child record grid edit based on other parent selection

I'm also trying to implement something very similar without much success in another area of my app. I've got a table "Inventory", a table "Sub-Inventory", and a merge table between them so as to give them a many-to-many relationship. (One inventory item can implicate the use of many sub inventory items, and one sub inventory item can be implicated in the use of many master inventory items). 

"Inventory items" is a parent table to "Pull list items", which is in turn a parent to "Sub Pull List Items", which is a child of a separate inventory table "Sub-Inventory Items." See relationship diagram below: 

What I'm trying to accomplish is a scenario in which a user can add additional Sub Pull List child records to a parent Pull List Item record, each with a parent Sub Inventory Item record. However, I don't want the user to have to scroll through every single Sub Inventory Item record; I want the possible parent Sub Inventory Item records to be filtered by matching the parent Inventory Item category (which is fed down to the Sub Pull List item child via its parent Pull List item).

The issue I'm running into is that I can't figure out a way to dynamically filter the report used for my grid edit override form on the "Sub Inventory Items" table by a value on a child record (the "Sub Pull List Item" record).

I've got a pipeline set up that automatically adds Sub Pull items to each Pull item and selects the appropriate parent Sub Inventory item based on the relationships defined between master and sub inventory in that merge table. But, I still need users to be able to add and edit additional sub pull items without having to search through every single sub inventory item every time. 

Does anybody have any ideas on this? Or know when QB plans on implementing type-ahead search in grid edits so I can completely avoid this whole thing? Haha.