The Item record should have a similar flag set coming through mod-inventory. Holdings records are not passed through mod-inventory at this point. As it happens, ui-inventory does not need the flag according to current drafts for the UX for the display of the bound-with aspects.
Back-end API for retrieving a composite bound-with record set
Eventually the UI should display all titles and holdings in a bound-with item. I would thus likely be convenient with an API in mod-inventory that could build such a response from the raw bound-with structure in mod-inventory-storage. The list would at minimum contain instance ID, title, and holdings ID for each piece in the bound-with.