The MiSTer FPGA Multi-Console IO board add-on allows for zero-latency analog video output via a Dsub-15 port. This port can be used for virtually any analog signal (component, VGA, etc) including RGB.
While you could use a cheap, modified VGA-to-SCART cable, they are far from ideal in a lot of ways. To answer the demand for a high-quality SCART solution, Retro-Access has launched a custom cable tailored to MiSTer, priced currently at $42. Get it here.
This same cable could be made with BNC leads as well.
Retro-Access MiSTer SCART Cable Features