I'm sure you've already given this a crack via some other technique (I just Cmd-F for it and didn't find) but I have had monitors with confusing EDIDs before that MacOS didn't handle well and the "screenresolution" CLI app https://github.com/jhford/screenresolution always let me set an arbitrary one. It was the only way to get some monitors to display at 100 Hz for me and worked very well for that since the resolution is mostly sticky.

Hey, thanks I hadn't tried screenresolution but that seems to simply set the resolution and refresh rate without controlling the scaling which is what is needed for configuring the HiDPI mode scaling.