You can also use an external program like StrokeIt, which permits you to define actions for both gestures and various combinations of mouse buttons. You can define those actions to be LS bangs, expanding the types of interactions permitted with modules like jdesk and hotkey. You can have actions take place when the desktop doesn't have focus, for instance. Of course, I've also defined a hotkey to show the popup menu (well, several keys and several popups actually) like DeViLbOi said - that's the easiest LS-only way to do what you ask.