The way to do this would be to first load up your FCP timeline in 720P with the MXO2. Under View–>Video Playback make sure that you choose a Matrox MXO2 720P setting either at 8 or 10 bit. The next step is to open the MXO2 control panel and take a look under the video output tab. From here you can choose to downscale all the outputs of the MXO2 by setting the Main channel to SD. You can also choose to downscale the analog and SDI outputs independently. On this same page you can also choose if you want to letterbox, center cut or just leave it anamorphic. Once you have set these options when you playback your 720P timeline in FCP you should now get a great quality downscale on your output.
Hope this helps,
Chris.