Just a few thoughts of mine to add to the mix. You see, I've been looking for an Android/Windows solution that replicates a Wacom Cintiq especially on an S-Pen-enabled device. And yes, such a solution would have to be pressure-sensitive to be of any real use to me. But, so far, I've not found anything that does what I want. The things that I've thought of in the process are as follows...
There are a lot of "remote" mice apps out there as well as remote desktop apps but none of them are pressure sensitive, which, in my mind, would be the easiest way to achieve the illusion of a Wacom Cintiq. I think the quickest way to achieve this is to extend VNC with pen support both in a VNC server and in an Android client.
About using WiFi versus USB. A lot of the remote mice work over WiFi and provide no lag mainly because they don't have to redraw the host's screen. But then lag doesn't seem to be that much of an issue with a lot of the remote desktop apps either, so I would say that an app that acts as a Wacom Cintiq clone is feasible. In fact, the quality doesn't have to be that great either, after all, the resolution on a Cintiq isn't very good, it is more there to provide a reference and some visual feedback to where the pen is; hence the proposed app could ask for a down-scaled version of the host's screen as an option for speed. In fact, as an additional option, the host could be made to transmit only the image of the active window or a predefined area of the screen.
In my opinion, having just some visual feedback on the surface I draw on is better than a perfect reproduction of the image on the screen. After all, my digitizer (a Wacom Intuos 5 touch with the wireless module) doesn't provide any visual feedback on its surface and yet people draw some amazing art using drawing tablets like it.
About writing a Windows driver, especially one that allowed the proposed app to communicate over USB. Yes, that would be the ideal solution but the easiest solution would be to write a little server application that sat in the tray on Windows and communicated with the Android app and Windows by sending Windows pen messages to the operating system. This is in fact what most of the remote mice apps' servers do (OK, to be pedantic they just send standard mouse messages, not pen ones, but a Windows message is a Windows message). Sure, some of these remote mice apps may come with proper Windows drivers but most don't. Why? Because, as people have pointed out, it's difficult to write one (especially with USB support).
The one flaw with the model of having to run some Windows software first (one that isn't a service) is that the software is unavailable on the login screen on Windows so one has to use the keyboard, but it really isn't such a big deal. I, for one, prefer to type on a real keyboard anyway.
-Andreas