The login script entry would be unique for each OU in the sense that the department specific deployment package would contain a registration key that will register the workstation in a specific OU.
So I will not import existing workstations created by ZfD6.5. I will use the login script to estentially re-register all workstations at the time the zcm agent is loaded. I will apply the same logic for inventory ( i.e. to just create a new inventory ).