the composite analog video stream are on different feeds.

You may need to go into the Panny's menu and select the monitor feed port (so you can display what is being recorded through the lens).

The b and a models should not be much different here...
ftp://ftp.panasonic.com/pub/Panasonic/Drivers/PBTS/manuals/OM_AG-DVX100B.pdf
See page 55 for selecting the video monitor out...

The only external firewire drives that might work with the Panasonic AG-DVX100 series (of which I am aware) are those from Focus Enhancements. Check the compatibility chart.
http://www.focusinfo.com/dynassets/documents/products/DTE_Compatible_Cameras_23Nov08.pdf