It's known that Darkmatter won't work well with OpenStep, since it can't understand drives that OpenStep has formatted.
It may not be as well known (apologies if it is!) but if you put your Darkmatter disk image on a second hard drive in your OpenStep system that you formatted elsewhere using NEXTSTEP, then Darkmatter appears to work fine. In one of my Dimension cubes, I have a SCSI2SD emulating two drives, and I've done exactly this external formatting trick for the second drive.
I didn't accomplish this setup in the easiest possible way: there was lots of time with the dd command in my case. But I don't think you have to do it how I did it. I guess the easiest way would be if there's a way to format a drive under OpenStep that still uses the NEXTSTEP layout. Does anyone know if this is possible?
Unfortunately, the copy of Darkmatter that I have still can't run Marathon, and that's a shame. (It claims it wants a display that can use "at least 256" colours; of course the Dimension works with only "millions of colours".) What is the most recent Darkmatter, anyway? My copy comes from the last time I was playing with it, which was probably 2018 or so.