Emm, wouldn't VNC do exactly what he wants?

Use PC, set up a dual monitor, VNC to the Maemo and make that full screen on the second display making monitor 1 the OS (Windows?) and the second the device. Did I misunderstand the OP?

"would it be possible to use your n900 through your PC as a separate window"

I'm thinking using the Maemo as a window, not using the Maemo screen as a secondary display. The N900 would run VNC server.