Sounds like you are hardware boxing, correct? Trying to get 5 PCs to display on a single TV-type display?

Svideo and BNC are both analog already, so even though you will get sucky resolution with this approach it shouldn't be an analog/digital conversion issue per se.

How are you getting from the svideo output on the PC to BNC input on the multiplexer? Something like this? http://www.svideo.com/svideo2bncmf.html

I don't know for your specific video card, but back in the day when I had a TV-out dispay adapter on my PC, I did in fact have to set the display properties to output signal on the svideo channel. Can you get one of the PCs to display on your 50" display using a straight-through svideo cable connection?