by using the webcam! when you hold your guitar both hands are occupied, but if you can use the head as mouse cursor you'll be even able to control the effect by moving your guitar up&down!
Hi. Really this kind of thing should be a project of its own where the program runs as a server with a MIDI output. Then the concept would not be limited to Rakarrack. It could be used for Hydrogen or ZynAddSubFX, or even as a simple mouse or pointer controller for presentations or educational purposes (like the smart boards).
Because video image processing can be a CPU hog, it would probably better if you ask the x.org developers to implement this type of interface.
Perhaps a start for Rakarrack is what Douglas McClendon has suggested in the past (guitarzyx guy), and that is a "whammy pad". It would be a big blank window that works like sort of a joy stick. You could map 2 parameters to some Rakarrack controls where one is X and the other is Y. Then a person could use a mouse, a real joystick, touchscreen monitor, or as you have recommended, video input.
If you can find a program that works with the web cam in this way and has MIDI output, then you already have it made :)