The screen may not accept it. It is designed to use a signal from a PlayStation, and since it outputs RGB (and RGB is better than s-video), it would make little sense to have the circuit needed for splitting the s-video signal.
Ok, a reply on the BenHeck forum seems to state that s-video is not possible on the PSone screen - it only takes composite and RGB signals.
Looks like, at this rate, the GBA mod is going to be the only real choice! Seems like the Nintendo systems are the best bet for composite - after all they connect to a home tv via composite, and the quality is excellent!
Anyone know of a different 5-7" tft screen which can take s-video?
Update - May '07: Ok, got system working properly on PSone with RGB, picture is stable (if using batteries not mains).