Trackball-like Mouse Drag

This is more a question for the mailing lists, perhaps, but here I am. Maybe a simple sample and hold or javascript problem, or maybe the feedback patch could do it, not sure. I'd like to keep accumulating mouse position, to attach to 3D Transform rotate, to have pseudo-trackball behavior. I can get the mouse origin to reset each time, no problem. I just don't see how to keep the last value, every time, to keep adding to the current.

I know dust posted some teapot rotating vector stuff, but his doesn't appear to accumulate axis position, either.