A high level test description is:

1. Configure both gpgsm and dirmngr to use OCSP.
2. Import the responder signer certificate with gpgsm --import.
3. Use a certificate with OCSP responder extension present, or configure a default OCSP responder in dirmngr.
4. Configure your OCSP responder to identify itself with key ID (and not subject name)
5. Attempt to sign or verify with gpgsm.
6. You should get an error, with dirmngr logs showing that the responder signer certificate could not be found.

Thank you for a quick fix (despite this being a minor problem).