There is actually a CSYNC scart cable at retrogamingcables (UK) that is wired up with the L/R RCA jacks:
This will route the audio right to the scart connector. I think you might be able to get MONO out of the same A/V port as the RGB with a different cable.
I run the Otaku 6 port Scart Switch (which has been awesome despite what I read about it online), I used to daisy chain x2 Hama Scart 3 port switches for a total of 5, but there was alot of noise and other video artifacts for every system I had.
Basically off my 6 retro systems, I either have breakout cables to separate the audio from any video right from the system (or the system has built in RCA audio out) which then goes to an 8 port RCA Switcher to my Receiver.
The OSSC was also pass it along just fine, but for my unit would add some noise and will randomly drop the audio of any game and any system for 5-15 seconds randomly. Though it’s actually been fine recently.