So long story short - until there is a version of MakeMKV-Bin which can be compiled for ARM processors this will not work on OSMC (but does apparently work on regular versions of Kodi 18.5)
Collecting some stuff for future reference:
[ Linux - DVD Automount]
https://forum.kodi.tv/showthread.php?tid=283756&page=2
Linux HOW-TO playback Blu-ray Disc in XBMC for Linux with MakeMKV (Plugin Addon)
https://forum.kodi.tv/showthread.php?tid=67420
Use libmmbd (MakeMKV) to play Blu rays on Windows
https://forum.kodi.tv/showthread.php?tid=189402
Linux How to associate “Play disc” with the Blu-ray discs?
https://forum.kodi.tv/showthread.php?tid=134828&page=7
Linux HOW-TO playback Blu-ray Disc in XBMC for Linux with MakeMKV (Plugin Addon)
https://forum.kodi.tv/showthread.php?tid=67420&highlight=makemkv
/r/Kodi - Makemkv - Kodi add-on (bluray)?https://www.reddit.com/r/kodi/comments/ejj2ua/makemkv_kodi_addon_bluray/
Makemkv Forum - Makemkv - Kodi 18 add-on/bluray decryption + playback
https://www.makemkv.com/forum/viewtopic.php?f=3&t=21219