OK, have resolved this thanks to comments from MarkG.
I had to create a custom EDID and force the Linux Drivers to use this instead of reading it from the TV, using :
Option "CustomEDID" "DFP-0:/etc/X11/samsung.bin"
I got the TV EDID dump from the Xorg.log, amputated the Extension Block data (anything after byte 127, 0 base), reset the Extension Block count to zero (byte 126) and incremented the checksum by 1 (byte 127) and, tada, the 9500GT now doesn't send the audio over DVI, meaning the TV now plays back the RCA audio.
//told you I was hardcore