Good to hear that the video quality is getting some attention. All the interference has certainly been my main annoyance with the otherwise quite nice s-video output.
You said that adding new features such as a SD card reader is not a goal here. However, if the layout is not yet finished, would you consider adding at least a header or some easy to access solder pads for the IEC bus and the voltage rails? That way people who would like to install stuff internally would have an easier time with the wiring. Maybe there are also other signals which would be nice to have easy access to on the motherboard.