You can use an OSSC, but I think it would be easier and better to use something like a ShinyBow SB-2840; that will do a simple colorspace conversion from RGB to YPbPr component.
Trying to use an OSSC and HDMI->VGA converter is just going to be way more expensive and complicated, and you’ll still need an RGB->YPbPr converter/transcoder, because a simple, passive VGA->component adapter like what I think you have isn’t going to do any conversion. The HD Retrovision cables have circuitry in them to convert RGB to YPbPr, and most VGA->component adapters will simply pass-through the original signals.
Also, speaking of the HD Retrovision cables, while HD Retrovision’s store is still closed, it looks like CastleMania Games currently has the Genesis cables in stock.