[otrs] ACL to validate customer-information

Hi, Some of our OTRS-tickets are generated through SOAP without a customer assigned to the ticket (ticket.customerid = NULL). I would like to force my agents to associate a customer to these tickets. Does anybody know how to write an ACL to check if the customer-assignment was not yet done or if a valid customer was found within the customer-DB to the given ticket.customerid. Bye, Alex