jonwolf..i don't think there should be any problem doing this. g5 to dvx via firewire, dvx to tv via RCA cords. Make sure of the following setup in FCP, View>External Video>All Frames, if you are watching from the timeline.
You may have to go into your A/V settings and then click AV devices and then unclick Mirror on Desktop and then RECLICK it. Go to VIEW in the top menu and select External Video then ALL Frames and your footage will pop up on your external. I have to go through these steps all the time anybody know why? I am not sure why I have to unclick Mirror on desktop then select it again for the computer to realize that is what I want it to do!