As long as the WAN is fast and relible you can run oprimized duplication over the link (which needs a Managed Backup Exec Server with Deduplication at each physcial location with the CAS also running Deduplication.
You will however have some issues with security accounts as this document ( http://www.veritas.com/docs/000075685 ) implies that you need a domain trust between the CAS and the MBE domain, and if your MBEs are in different customer's domains then juts on security principles, I doubt you wil be allowed to setup trusts from your single domian to all of the others. So wil probably need a CAS for each customer (added to their domains to make it simpler) - you could then remoet conteol these CAS servers fom your own systems to make admin relativly easy.
As an aside have you also thought what wil happen if for instance one of your customers decides to discontinue using your serviices and insists you return/delete any of the offsite backup sets... if you share a central dedulication location (or even worse an individual tape) between customers you will not be able to delete the leaving customer's data without having an affect on the remaining customers' data - again a very good reason to keep each customer segragated and not not try to centrally manage across customers from one CAS.
