FDW, replication and CREATE SERVER - will it work?
You cannot really use postgres_fdw to connect from a replica server to the master, because you cannot create the foreign server on the replica, it is read only.

You'd have to run the create on the master, then rely on it replicating to the read-replica.

Even if you were to do this, it would make no sense. There is no point connecting from a replica to the master, because the replica has the same data as the master. There is no purpose to doing this.