Hi DanneS,
You can make the custom cable using a VGA monitor cable (one with BNC outputs would be easiest) and then wiring the RGB cables to a 9 pin mini DIN connector. You can get the correct 9 pin mini DIN from RS Components in the UK.
If you configure the X-Meridian for 6 channel output, TMT3 will decode any selected soundtrack including TrueHD and DTS Master and output to the soundcard (48KHz, 16 bit max). If you have the X-Tension add-on you can then take the output from the G9 connector to a Meridian multichannel input.
Although the X-Meridian supports 8 channel output mode you would have to use 6 channel mode into a 568.2mm or a G series processor.
The bad news is that the X-Meridian soundcard was discontinued 2 years ago when the manufacturer lost access to the main sound chip, so finding one isn't so easy.
You also have to make sure you can get hold of an X-Tension add-on card to get the G9 connector. This card actively does the SPDIF output, you can't just connect to the internal connector on the soundcard.
-Ronnie