I recently bought a Barco ADVM20 studio monitor with a whopping 1000 TVL lines count

This is a 20 inch CRT monitor that comes with SDI, composite and S-Video inputs.
Unfortunately, Barco omitted discrete RGB inputs.
There are probably analog RGB to SDI A/D converters, but this would imply another
internal D/A conversion step.
I am really looking for a way to add RGB inputs to this monitor.
My guess is that the SDI signal is processed to R,G,B anyhow somewhere in the circuit.
There are many people out here who have successfully modded CRT TV sets and CRT monitors.
Does anyone know if the Barco ADVM20 has been RGB modded before?
Can someone point me to the schematics and instructions? Google did not turn out any results for me.
Many thanks in advance.
Kind Regards from Belgium,
Christian