Alas, Bunsen is correct about the monitor requirements. Back in the 1980s, it was very common for a secondary supplier to modify an OEM monitor. Many of the big Mac and Unix workstation analogue monitors were modified Sony SDM Trinitrons. Video standards barely existed, and it was easier to modify the monitor than to modify the display adapter. If you check the old adverts, you'll see how "standard video support" was so much of a big issue.
Power R created an adapter that converted a compact Mac video signal into TTL. Even the horizontal rate remained weird Mac-ness. Given that Power R were a big Mac developer at the time, we can assume that others modded monitors for it.
Note that although three RGB pins are connected, the adapter is for the SE and SE/30 which do not have built-in colour support. I presume that the connections are to help the connected display to sync. The SE/30 has Color QuickDraw, but requires a PDS adapter to display even greyscales.
Keep looking, as I do, for weird and wacky compact Mac monitors and video cards. To follow this project further, look out for one of the ancient NEC Multisyncs or similar, as suggested by Bunsen.
Redrouteone suggests that contemporary TTL monitors are available. This is true and Microvitec are one in Europe. Their Desk-Scan 1402 sounds like hacker's heaven all over again, but the input modifiers seem to be additional extras. I mention this monitor for one reason: there is a continuing market for old spec monitors, met by specialists, in order to maintain old medical and industrial control equipment.
Mac128, look for a monitor on eBay in the specialist categories. TTL with wide frequency and line tolerance, but as close as possible to the the Power R adapter. And then ask here.
Abstract question: Connecting a Power R to an NEC Multisync in analogue mode? Unless the TTL voltage is too high, it should display a viewable picture in strange colours?