Is it possible to set which material output node is used? It is possible to have more than one the Material output node. As far as I have seen, only one of them will be used. Probably the first. Can it be changed?

I often use a second simple diffuse shader to get the output of a node in the middle of the network. Kind of like a viewer node, but with a material. With a simple way to switch the active output node, it could be made lots smoother.
I bet there's no way as simple as Greg's addon, currently. If you still chasing for another possible answer, try using Add Shader, hit M to mute one to show another. (Select them both then hit M will do a quicker switch)



Personally, I would recommend Greg's addon. Whatever, if this is what you want.