Ok, I'm using my keyboard as a guitar, let's use the mouse as a pick.
How would it work:
The "mouse pick" would be triggered when the mouse speed reaches a top value, and would be released when it reaches a minimum value. This would work as a third "pick button".
This would be also great for touchpads on laptops (it would require a "horizontal game mode"). And it would be great that the mouse could also be used in the menus (instead of up/down and enter).
Possible problems:
* The surface: Optical mice might not work well depending on the surface you are using it (usually, the clothes you are wearing).
* False picks: using a device such as a mouse isn't as simple as keyboard keys: some chords might be missed and "mouse pick" could be triggered when you didn't want to.