What I can suggest you would be to modify the NonEditableComboBox Template in order to replace the ItemsPresenter inside with a GridView. Actually I have such an example which demonstrates the exact approach - the selection is working and SelectionBoxTemplate can be applied. Please check the attached sample project and let me know whether it works for you.
I'm looking forward to hearing from you.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.