Our school seems to have a habit of employing people with the same first name! I think i would go down the route of numbers as usernames.
What application do you use to create your accounts from SIMS.net? If you could add the user's local id (admission number for students) from SIMS to a active directory field you could then have a custom plugin written that extends the moodle ldap plugin to search for this id before creating a new account. If it finds the id it can rename the moodle account.
Have dropped you a PM.