The xorg.conf file would be located in the /etc/X11 directory HOWEVER most of the current xserver-xorg implementations (at least the open source ones) probe the hardware at runtime and don't use an xorg.conf file at all
That being said, if the file exists it should get read in addition to the hardware probe, so you should be able to create one and put any custom configs in there - for example on this machine I have:
which suppresses the proprietary Nvidia banner.
$ cat /etc/X11/xorg.conf
Identifier "Configured Video Device"
Option "NoLogo" "True"
# Option "UseVBios" 0
[Sorry I can't help with the specifics - I *do* have an old Thinkpad X30 with IBM graphics but I never tried to get S-Video out of it]