I explored using the tuner to assign to the middle mouse button and noticed something interesting.
The Twiddler.cfg Version 5 Binary File Format doc says the mouse buttons occur in the order: left right then middle.
However, after assigning Left GUI to the middle mouse button in the tuner,
the tuner generated a twiddler.cfg binary config file with the buttons in the order left, middle, right.
And after loading the twiddler.cfg, the middle button was mapped as assigned by the tuner.
Just thought I'd mention it for those that may look at the binary format.