i have done shows where the led screens are run from the composite or s-video output from the 9800.

composite looks a bit shit around the edges, but sometimes its fine for what the client is aware of.
You just have to check.

You can output composite or s-video directly from the mac using the retail 9800, and catalyst will work with the outputs.

What you must do is actually have a video device connected to the system when you boot it up.
The 9800 cards senses the output on the composite connector, and if it doesnt see a 75ohm resistive load, it assumes no video device is connected or present, and does not give you the option of outputing composite video.