I'm surprised those adapters seem to be so easy to find as I'm pretty sure Multisync monitors stopped supporting the TTL video standards (MGA/CGA/EGA/etc) somewhere around 1991 or so. I've owned two NEC Multisyncs: an oddball "Multisync GS", a grayscale version of the original Multisync, and had a 9 pin connector on the back. (It of course required a 9-to-15 pin adapter to connect to a VGA card.) I also had a "3D", which had a captive 15 pin VGA cable but *allegedly* could take an adapter on top of that to interface with TTL video cards, but I never used that capability. The thing I believe that *all* the "Old School" Multi-mode Multisyncs had in common however (whether they were made by NEC or a competitor, like Mitsubishi with their "Diamondscan" line) was that there was a switch on them somewhere that selected whether the video used analog or digital signaling. (It was a big obvious slide switch on the older Multisync, while toggling the "mode" button apparently did it on the 3D.) A little Googling suggests that the 3D was the *last* Multisync to support TTL. That could be wrong, but I doubt the capability extends much further.
I suspect you're asking for trouble hooking up one of those nine pin adapters to a newer monitor lacking that switch and feeding what's *probably* a TTL signal into it.