Instead of plugging a mouse into the touchpad (I guess that's the current workaround), why not turn the tablet into a touchpad when mirroring the display of the tablet on a large screen? With that setup, the tablet display wouldn't actually be mirrored, the big display would look like a desktop computer display with a pointer floating around on it. Also, the virtual touchpad on the tablet could be resizable so that you could simultaneously show a virtual keyboard (neither of which would show on the big display). Or there could be a spot on the tablet display tapped to switch between virtual keyboard and touchpad. Again, neither showing up on the big display.
Let me know if I'm not being clear enough. This has nothing to do with simply mirroring the tablet display to a monitor/TV, I know that can be done through MHL and HDMI stuff. Also, it has nothing to do with using the tablet as a touchpad for another computer. The only components are the tablet and the big display (monitor or TV).
And of course if it's already been done, please let me know.