Maz
Are you honestly telling me that a Customer that books a holiday of any kind via a travel agent (independent or any other kind ) that the only people that have full contact details of that customer is the agent that booked it I don't think so. Firstly if the holiday involves a flight the airline will have the customers details via Passport details secondly most hotels or accommodation will have to have customer details.
Yes i can understand independent Agents wanting to keep these customers but if that said agent gives good quality service to its customers then the customer WILL come back year after year. And I am sure that the customer will not mind if there details are shared because surely the more people now the details the better in case of an emergency.

I remember selling holidays with numerous tour operators, and at the end of the trip the tour leader or rep would give all the customers a feedback form to fill in, with a place for the name, phone number and email address. So often customers would come back and tell us that they had received a communication from said operator that if they booked direct next time, they would get 10% off, or an alternate reward.
So, unless they're a cast iron guarantee that this won't happen in your agency agreement, I wouldn't go down this route.