So it would be great to generate the HASHVALUE with a stored secret together with the ticketnumber. On the feedback system there you can prove the hash with your secret and accept the feedback score.
Requirements in zammad:
store a secret
generate a hash with the secret and ticket number
possibility to insert this hash in a ticket (maybe as an object variable).
I think it is not possible to create a hashvalue in Zammad itself. You could set this value throught the REST API or use an automation service. We use Integromat for this. You can trigger your Integromat scenario via a webhook and the update the ticket with the hashvalue.