I don't think your idea will work, but hey, it's worth a try.
Also, I noticed that the SelectionBox class resets scrollFactor to 0 every tick in its update() method... So if you haven't changed it in there, that's worth a try too.
(oh, and a possibly stupid question: What exactly is the SelectionBox.target you check for overlaps with? It doesn't seem to ever be assigned a value :confused:)