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..)