Active grabs are problematic when applications hang.
Let me explain a bit more my idea: A "private" zone would correspond to parts of the client window. Clients should only allowed to request such zones on the windows they own. Because of that, this would really not apply to touch pads, as they are "relative" input devices and do not map directly to screen coordinates.
The utility of a private zone would be to capture touch events (not gestures) initiating in those zones. Touch events initiated elsewhere should not be affected, even when crossing private zones. Of course, an application should not be allowed to "privatize" the whole screen (even if shown full screen).