"A solution for such cases would be to define "private" touch zones. Any touch initiated in such zones would be handled exclusively by that application."
Sounds fine for a touchscreen but it doesn't make as much sense on a touchpad where even a small "private" zone would be a poor use of the small area. I think something similar to a keyboard grab -- except for touch input -- might be suitable for the latter. Yet keyboard grabs can be quite annoying if/when applications fail to release them. So I'm not sure what else could reasonably be done to handle the need for "private" input.