I have one of these devices to use the N900 on monitors.

Using it as a 2 nd monitor should be possible, since mplayer used "xinerama" and you can see a video output to the TV and see the emails in the N900 screen.

I hope you get to make it possible for any application.

One idea is to use the media player controls on the screen of the N900 and the movie on the TV output.