According to this site (http://www.amiga-stuff.com/hardware/), the A520 not only relies on a clock signal from the Amiga, but it also uses the MC1377 (http://www.amiga-stuff.com/hardware/mc1377.html) chroma encoder- not exactly worth the effort.
I also found a chip (http://www.amiga-stuff.com/hardware/v7021.html) that converts composite or maybe S-Video into RGB, but it's a Sony chip, and the only datasheet I could find is in Japanese.