I once had a look for the opposite thingy, splitting composite video or s-video into RGB, and I think while surfing the net for a solution I found various posibilities for what you are searching for.
I will have a look if I have the sites still bookmarked or downloaded some schematics.
Converting RGB into s-video is for sure much easier and cheaper to do than splitting video into RGB !
If you can live with composite video than you can also simply get an A-520 modulator, which has a composite output, too ( at least the PAL versions
)