I think you may have brought me to realise a limitation. I thought we could expose an item’s children from the junction table at the item level itself, and therefore have a recursive relationship, but none of the lookup, formula or auto-relations fields allow this.
I was thinking we could have the [IR:Children] field be a formula of [BOM Lines as Parent].Collection([Child Item]) to return a collection through BOM Lines, for all child items where the current item is a parent. But this is not possible, since the Collection function is not available.
Hopefully @Chr1sG can help shed some light on this.