There was at one point a demonstration QuickTime app that would do this; live-convert any video source to a dv stream and send it to the firewire port for display on a camera or external source. Sadly, it caused panics even when it was new and is totally dead now, so there aren't any cost-free options that I'm aware of.
Personally, I have an ATI video card that supports two displays, and I bought Apple's little $20 dongle for converting the DVI out to an S-Video signal so I just have my TV set up as a seperate display and I can put anything at all over there to watch it on the big tube if I want--Real Media, QT Movies, iMovie previews, even the DVD player (which I've used on a couple of damaged discs that the Mac would read but my DVD player wouldn't).
Another option is to use iMovie or QT Pro to convert the video to a .dv file, then use iMovie to play it, but this is time and hard drive space consuming, and certainly isn't a "live" solution. Some of the DVR-type boxes that are meant for time-delaying TV may also do this, but they're going to cost you some money to get started.
