The Add OpenAccess Service wizard is simply a tool for generating a service, should you choose to you can always move the generated DTO classes (or interfaces) in a separate project/assembly. It will be a little troublesome when updating the service using the wizard but is still the way to go.
There is also something else, once you have a service generated and running, and simply add a reference to that service using the Add Service Reference option in Visual Studio, by default you will have created proxy objects on the client that you will use. There isn't a need to reuse same interfaces or DTOs on the client, given that similar ones will be created for you. So a customer using your application will only be able to decompile the proxy classes that are exposed from your service.
I hope this is helpful, do let us know if you need something else.
the Telerik team
OpenAccess ORM Q2'12 Now Available! Get your hands on all the new stuff