Item relationships by Operating Unit

We have a business need to have different superseded items per operating unit - so the item that replaces item A is item B in the US operating unit, but the same item A is replaced by item C in our European operating unit.  When trying to use item relationships to accomplish this, we found that item relationships can only be loaded at the global level.  When going into the item relationships form, regardless of the Org you choose, the form points to the item master organization.  If we had multiple item masters, then it would be possible to load