Hi Tom,
Sorry but I never had to implement such requirement, so I don't know what the system behavior will be. I can only guess that if you provide a BP number in CT_PARTNER_KEYS for which a relationship is entered in CT_REL_PARTNER_KEYS, then only this relationship will be shown. And if you pass a BP number in CT_PARTNER_KEYS only, then all(or no) relationship(s) will be shown... but again: that's only what I would expect, I never tested it. Your feedback will be very interesting.
Cheers,
Nicolas.