There have been a few flying wing gliders that use dihedral and rudder-elevator control. The Raven is one and the designer of that model has a few others like it. The Windlord is another. Plans for that one are available from RCM. A buddy of mine built one and it flew great. Large rudder though... VERY large rudder.

Trainer? A properley designed, built and trimmed flying wing would be fine as far as control response is concerned but perspective to determine what the model is doing in the air would be MUCH more difficult. Conventional designs just have much more bits that can be used for visual cues to allow the student to see what the model is doing at some distance. So while it is possible I truly don't think it's a great idea.