cool project.. why not just use your tv's RGB input, it has similar circuitry already built in?
To avoid artefacts due to the internal A/D converter i assume. Is the shown Turrican in 576i? Looks horrible in motion. Direct HDMI without extra A/D convertion would be awesome. But as Jope said. 720p/1080p in 50Hz should be selectable. There are gazillions of HDTV monitors/TVs which supports 50Hz via HDMI. Nice project btw.