Map caps-lock key to middle mouse click

If you first remove the lock modifier from the Caps Lock key you should then be able to assign the key in Keyboard Shortcuts, use the command below to do that:

echo  'remove lock = Caps_Lock' | xmodmap -

Another easy solution if you prefer to use a GUI is to install easystroke:

sudo apt-get install easystroke

You can easily map keyboard keys and shortcuts to mouse buttons as well as mouse gestures.