I have been using the clicker outside for wees and poos. I tend to say "be quick" as he's weeing and then click. He's never stopped a wee because of the click. After he has weed he then comes to me for his treat! He sits and looks at me as if to say "I've been a good boy mummy!"
We use clicker training for other things as well so he knows that the click means he's done something good. I will say though that just because you're using the clicker, it won't necessarily mean that toilet training will automatically get quicker. It's just something that comes with time, it appears to be normal with some pups to progress and then regress