About the AV booster,
There is a very very good posibility that if it is modded for RGB, then RGB is the only thing that will output. The standard composite (yellow) plug is most likely not getting a signal.
Unfortunatly, i cant offer you any advise about the inside of your PC Engine. I have only owned Duos myself. However, i have heard that the PAL PC Engines have extra hardware in order to produce a PAL signal. This is what it looks like to me.
Try posting these pics at the GAMESX.com forum if you dont have any luck here.