My experience has been that every 5-bnc multisync I've found has worked
with sync-on-green, however my experience is limited to Viewsonic and NEC
monitors in this regard. I would agree with your guess that the monitor you
looked at mixed sync on the read and blue inputs.
Do not be scared by the prospect of building a sync separator circuit. The
chip from National is easy to use and can easily be hand wired. You can
power it from a 9v battery and when the screen goes away change the battery
or use a "wall wart" type system with an on board regulator.
--Chuck