Ehm, and how do you expect that plugin to work? Capture your desktop sound output and visualize it?
Its definitely alot easier and far more viable to just window capture Winamps/Foobar/VLC's visualizer. But hey, if you find someone that wants to do it, we wont stop him :)
You can also do it all on one monitor unless you use a fullscreen application, or you can connect a fake monitor and capture it from there.
Screen capture is a very clungy way of doing visuaizer, as when you maximise other programs like VDJ or QLC you overwrite the capture, or you lose significant screen space. To me, the only way to do that would be to have a Video Router, to capture the output of the application (like VLC etc) without it going to a screen. Such a router would be very useful for several applications, but I don't know of any programs to do that, does anyone?.