If your dog does it on his own then you can reinforce him and he will do it more often, and then you can add a cue. But I haven't found a way to initiate this specific behavior with my dogs. If you find out how please let me know!
The problem I have is that I have always discouraged that kind of behavior with my dogs, ever since they were puppies.
So I don't think I will be teaching this. Unless one of you wants to record a video I would be very happy to put it up on the website. But keep in mind that you still need to figure out a way to initiate the behavior.
However, I think it would be hard to reinforce the dog only for smiling and not for his defensive behavior.
What I do with Ellie is make short little blows in her face, almost like you're saying "pup pup pup" really quickly.
In Ellie's case, her natural reaction is to pull her lips back slightly which shows her teeth, while slightly lunging forward to "catch" the breaths . All I then do is click and reward at the right moment. I shall end up calling it "show teeth".