Consider using these identifier types:
DC Death Certificate ID The identifier assigned to a death certificate, and printed on the death certificate when issued by a jurisdictional vital records office
DCFN Death Certificate File Number The identifier used within the jurisdictional vital records office file system as an auxiliary means of accessing the record associated with the death certificate.

From https://terminology.hl7.org/2.1.0/CodeSystem-v2-0203.html following the pattern in BFDR. ALternatively, have BFDR follow VRDR pattern.