I've now extensively tested this with boxtron / dosbox / dosbox-x. Within dosbox-x and dosbox, I can run the DFORCEDH.EXE program, and set the mouse control, save and exit, and then launch DARK.EXE and have a crosshair and can look up / down.
On the other hand, if I then try to launch (from the same Game directory as the Steam install) with Boxtron, it stomps over whatever change there is, and breaks mouse look. Though somehow the crosshair hack remains.
With copying out boxtron's configuration and tweaking it so it first launches DFORCEDH.EXE in case I broke it by playing through Steam, then it will launch DARK.EXE with the patch.
So included is the script, config file, the dehacker utility, and the config file used (mostly the same as the one generated by boxtron, but copied the mount and run commands over).
SWDF.zip