The HL7 registry specifies OIDs for identifying passports (root 2.16.840.1.113883.4.330) :
2.16.840.1.113883.4.330.[country oid code]
I strongly suggest that a rule for building URLs for the identification of passports will be specified as well.
This pattern should be included in the identifier registry page.
A possible pattern could be http://hl7.org/fhir/sid/passport/[ISO alpha-3 country code])
e.g. French passport http://hl7.org/fhir/sid/passport/fra
(unless someone else has already assigned an uri for this..)