Filtering custom list field by a null value

I've got 2 custom records; Insurance Carrier and Insurance Type

Entity records have 2 custom fields, one of each type above.  This allows me to specify the customer's insurance carrier (EG: Medicare, Blue Cross, etc) and their insurance type (EG: HMO, PPO, etc)

Insurance Type records have "global" settings for a given type.  Insurance Carrier records have optional child Insurance Type records, these act as an override for the entities insurance type selection.

For example:
Sale rep enters customer "Brown" who has blue cross of New York HMO.  Sales rep doesn't know that Blue Cross of New York HMO plans have specifc, non-standard properties to our business.  So we have created a child Insurance Type record on the Blue Cross of New York record with these specific settings.  These settings override the generic HMO record that the rep assigned when creating the patient.