To wrap this up, it seems that this is likely to be a problem with the way this particular bluray disk was authored. It’s not a commercial disc, and isn’t encrypted, but it contains a structure that confuses Kodi (has an AACS folder), and so it’s not really OSMC specific.
In the end, I used MakeMKV to extract the MKV files, and abandoned trying to play it as a bluray. That suited my purposes well enough, though the bluray menus and presentation is really well done.
Copying the files from the mounted ISO into a folder, minus the AACS (encyption) folder, did not work. It played the intro sequence, but got hung up in a couple different places thereafter and crashed. Log for the curious.
In the process, I learned about what’s required to play commericial blurays on OSMC/Kodi. (See sam’s post above, with link.) In theory, the right libraries are already installed by OSMC. WIth a properly installed and current keydb.cfg file for the keys, Kodi should be able to play a bluray .iso file.