Custom LOV for Supplier Contacts on PO_POXPOEPO

Has anyone created a custom LOV for the supplier contacts on the purchase order form PO_POXPOEPO? Are we even able to do this? The seeded LOV pulls in user_name -- not seeing the value of this field. Would make MUCH more sense to pull in email address. But I'm not having any luck getting a personalization to work -- the form doesn't seem to recognize it, so I'm wondering if this LOV can't be customized.