What platform? On screen graphics *is* a portability issue. If you are willing to use jMax there is gridflow to be mentioned.
Under linux i am currently writing on a bitmap external which could learn to use X11 (but until now it only speaks PNG). Think this will be nice for feedback. To know about mouse clicks in the feedback window would also be nice.
You want realtime screen shots?! (better shooting a window ;) - Now i'm astonished. It is quite complementary to putting a window on screen so implementation is quite probable but will need about two weeks. Hope i got some webspace until then.
My initial idea was to use bitmaps as sequencer format (to use gimp as sequencer). But waveshaping and fader control are also applications.
seems like this is similiar to the kinds of things i want to do with it, i think you got the idea. i'm gonna get a new (old) computer from a co worker and i guess it would be good to install linux on it to help with this sort of thing. im running windows me right now