I'm not sure how strict NSW BDM's are? As John is only after a birth date would it be worth John's time to email BDM's and indicate to them he has Roy's Marriage Cert, Roy's war records, and details of Roy's death 2O years ago.
I just looked at the BDM's site, and and although births of less than 100 years are restricted except to the individual himself/herself, the wording does read "
records are normally only available to the person named on the certificate"
Does "
normally" suggest not all the time? Roy has been deceased for over 20 years, and John has a few of Roy's other records.
No harm in trying

Les