Stuey told me about this thread so I have registered, apologies for not introducing myself first, I am fairly active on some other forums which I wont mention since im not sure if its against your board rules yet. I have lurked here a bit too.
I am the person who modded Stueys PC Engine as shown above for RGB output. I think the original poster may be wanting RGB output instead of A/V output, but yes, you could call the socket an "A/V socket" that outputs RGB..
I modded Stueys by taking out the old RF unit and trying to neatly enlarge the hole to fit the new DIN socket.
It would of been possible if it were a standalone PC Engine to have a captive cable coming out of the rear i.e. no socket, just a cable coming out secured by a grommet, but since he wanted to use it with an IFU-30 and CD-ROM unit that wasnt an option.
Anyway if anyone wants to get in touch I can provide plenty of feedback and references with regards to my modding skills, all happy people of course, just send me a PM and I can provide all the necessary info.
Ive only got into PC Engine recently, ive played "teh romz" some time ago on my GP32, and I really like the system and games. Im a big collector of Sega and Nintendo, from the UK, North-West (Lancashire)
Anyway peace out, James.