You could get an Extron Crosspoint and run all your RGB capable consoles to it, then run the OUT of the Extron Crosspoint in to the OSSC via a BNC to SCART cable.
Extron Crosspoints are big matrix switchers. Matrix meaning that you can have multiple in to multiple out.. It can be thought of simply as a switchbox that can run to your OSSC.
The little hurdle here is surfing eBay to get an Extrton Crosspoint, but they do seem quite available as of the time of this writing.
If you’re still unsure of cables, what is RGB capable, etc.. I highly recommend watching the RGB Masterclass Playslist on the My Life In Gaming YouTube channel.
If you live in Canada, Leon Kiriliuk is a great modder who will RGB mod your N64. He did mine and couldn’t be happier with the work. He even added a JTAG connector on the RGBN64 Board so I can update the firmware whenever its needed!