I spent a few hours this weekend tracing the composite out to expansion port line on a TurboGrafx 16 (as you suggested Lawrence). It was a lot easier than the PC Engine or DuoR, as all the components are clearly marked. I am 90% sure i have everything correct, but could have made mistakes. One thing to note that pin 30 is not the composite line, it just goes to ground through a capacitor like pins 31, 37 & 38.
I Reproduced the circuit on a test pcb, and it still doesnt work on my PC Engine GT. I think maybe some of the pins such as those above may need to be hooked up, as they arent on the GT. I was just too burnt out to continue any further on it for now.
Here is my shitty "schematic" (and i use the term loosely).
(http://files.myopera.com/grahf/albums/3639/hu6260%20composite%20trace%20small.jpg)
Fullsize picture:
http://files.myopera.com/grahf/albums/3639...ite%20trace.jpg (http://files.myopera.com/grahf/albums/3639/hu6260%20composite%20trace.jpg)