Setting up a custom transaction line field to show bills of materials for that item

Hi everyone! I'm trying to add a custom field to our sales orders that allows our ops team to assign a bill of material at the sales order stage. We have standalone bills of material, and some of our items can be built in a couple or three different ways. The people who know which way these items should be built are the people who are making the sales orders, so I would like to assign a bill of material at the sales order level.

I've created a custom field with the type list/record bill of materials. This makes the option to enter a bom appear on the sales order and a BOM can be selected and linked to the transaction line. However, I would like to filter the list to just the bills of material available for the item on that line. I haven't been able to figure out how get at that information. If I set the source list to Item, the list of BOMs is not an option. Bill of Material is not an option in the source list. When I look