No image on New Net City/Toshiba Chassis

[invzim]

is there a standardized spec for this, or do this or is this just an undocumented quirk/feature of the monitor

Looking at the tri-sync helper page, it looks like there's an option to output CSync on pin 13 or HVSync across pin 13 and 14 respectively, so maybe that is the standard way of doing it? Perhaps @invzim knows if this is typically supported by arcade tri-sync monitors.
https://irkenlabs.com/jammafier/tri-sync-helper

The tri-sync helper (rev 2.0), has an option to output CSYNC on pin 13 while leaving pin 14 (vsync) unconnected. The reason for this is some arcade games have missing hsyncs during vsync, and that one sanwa pfx chassis didn't like this when using H/V sync separated but was ok with it if it was fed this as csync only. AFAIK all tri-sync monitors can handle H/V sync, and I'm sure the csync out of a mame setup does not have missing hsyncs during vsync.

Would be easier to explain with a scope grab, but I don't have any handy.

 

[nem]

I'm ultimately wondering if I can get away with rewiring a regular SCART connector to a DE-15 pinout and have this work without using some extra circuit to convert RGBS to RGBHV.

You can absolutely do this.