Making adjustments needed for this test while creating this post, but does anyone know if this is possible? Are the registry entries the only place the manager address(es) is(are) stored?
Yes, it is just those 2 registry keys.
And i don't know if the group policy settings are applied before the App Volumes service is started.
No, i don't think so to be honest. Also the Appvolumes agent send a message to the manager the moment it starts up before a user even logs in.
You could try running a startup script that changes the registry keys and restart the service after this change. It could be that the machine then reconnects to the new manager, not 100% sure though.
Do keep in mind that the test machine will also be registered in the production database as it will call into that during startup. Twice per day the manager checks to see if machines in the database still have stale attachments. It could be that those appstacks are detached by the production Appvolumes. If this is just a test it could well work for you but do keep this in mind,