OpenPGP/X.509 bridge: how to verify public key?
If you can put the entire PGP certificate in a proprietary non-critical extension then you don't need to find the PGP certificate in a store. This solution depends on the condition that you are able to create your own OID and insert the PGP certificate in the extension. Furthermore, the server should accept such a certificate and contain methods of validating the PGP certificate as well.

The CA can accept or refuse requests containing a PGP certificate. It should at least check that the key in the PGP certificate matches the one in the signed request. One method of doing this is to validate that the modulus of the PGP and X5.09 public key match.