My laptop touchpad has left button (for mouse click 1), a right button (for mouse click 3). To get mouse click 2 you must simultaneously press the left and right buttons. Will Unity let me reprogram this action to. say, Alt-Z (or some other key combination)?
This is probably not suited as a feature for Unity - it's more related to configuring the underlying X11 input devices. I'm not sure how to do it, but I am certain there is some X11 feature that lets you do this, an there probably is no nice GUI for it so I hope you don't mind text config files or scripts :-)
It seems someone on stackoverflow already have solves this problem maybe... https://stackoverflow.com/questions/294177/is-it-possible-to-map-a-key-press-to-simulate-middle-click-in-xwindows