You should be aware that these security monitors require a "composite input". While obviously they don't require a color composite signal (which is often implied when calling a signal "composite"), this input requires the video signal and the sync signal (both horizontal and vertical) combined on that BNC input. So you will not get around some kind of wiring adapter (possibly just mixing green, hsync and vsync using resistors) to make a security monitors work on a TTL graphics cards like MDA or CGA.

Good point, synch combiner probably required. I guess I got spoiled a couple of years after that when I was messing around with Tomi Engdahl's VGA2TV and the Trident and CL cards I had did synch on green.... Then I have been spoiled the last ~20 years with dirt cheap monitors and forgot all the details of the video class monitor hacks.

I am trying to dredge it all back up now for pre-VGA though since monitors for those are super hard to find. I have one mono and one color, but I've never seen anything like a KVM switch for pre-VGA/TTL, though I guess since it is TTL then something with 74 series logic shouldn't be too hard to work out. Another outside possibility is that I have spotted one or two LCD with TTL input... or needing a 3.3V or 2.5V level converter, but all the same, digital.

Like the mentioned sync combiner, you adapt the TTL etc. video signal with a few additional resistor(s) too. The arduino people who bitbang composhit outputs use a simple two resistor method, 1k on sync and 470ohm on the luma signal, with 75ohm in the TV/monitor completing the circuit. This works if you have a Csync available, with separate HV sync you're gonna need another resistor if you can get positive Vsync and negative Hsync and if you need to combine the other color channels into the luma too, you'll need 2 more resistors. I cannot comment on the exact values, since this isn't something I have had to do myself but in the end you'll need to get sync pulses stay between 0...300mV and video signal part from 300...1000mV for a compliant video signal for the monitor. Video can go higher but eventually it'll clip the input stages and then you're gonna get trailing artifacts as the transistors involved recover from saturation.