I asked this once and had an email from M about it. I can't find it to give the official response but it would appear to be dependant on whether the synch is part of the signal (normally on green) or not. However, the bit I can't remember is the bit about what happens if synch is not on green and is a separate signal (as per SCART). Whether or not this can be satisfactorily switched using the composite inputs I can't remember - its going to be dependant on if there is any skew in the switching of different types of inputs.
I never bothered in the end so didn't get a chance to experiment. I also looked at a RGB to YUV converter as VK mentions - they are of variable quality and the most highly rated one was from
js-technology. I was offered one once at a discount and regret not buying it as its quality was very very good.