By "...and not try and update a notes installation worldwide" do you mean to not replicate changes made to the directory (names.nsf) replica on the spoke server out to other replicas in the domain? If that is what you mean then you need to modify the replication scheme of names.nsf from the spoke server to other replica(s) to NOT replicate the design elements installed by DUC (specifically csAdmin install pieces) and possibly the few extra fields Unity writes to person and group records when imported into Unity. This information can be obtained from Lotus and Cisco.
You can configure Unity for Domino to deliver messages and gather directory information all from the spoke server if desired. You need to:
1) install Notes on the Unity server
2) configure Notes on the Unity server to use an account whose mail file resides on the spoke server
3) During Message Store Configuration Wizard, specify names.nsf (or whatever directory database you use) on the spoke server.