Out of curiosity, I set up an old laptop with a minimal debian system just sufficient to run an X server and usb networking. I created a cloned set of Easy Debian scripts which connect to the laptop's X server over the usb0 connection. The result is totally satisfactory. There is no delay on keyboard or mouse input, and the screen updates smoothly. The N900 continues to use the wi-fi or 3G connection for all other network access.