Found a fix here at InsanelyMac (thx @lubeats) for loss of DVI 2nd screen; patch of AppleGraphicsDevicePolicy kext (a bypass for some checks, details elude me now, it was a quick read).
Find: BA05000000
Replace by: BA00000000
With this patch in place, I recovered DVI output off my docking station in Sierra 10.12.6 and High sierra 10.13.6.