I don’t fully understand what you’re asking but I’ll try to answer. As always, it’s not a choice to support or not support S928X-J, technically, at the moment, it’s not possible. Regarding S928X, of course it’s not supported, it’s not DV licensed.
If that‘s how it works e.g. on AVDVP builds then yes. You can use L5 and access OSD.
Almost all UHD-BluRay remuxes have active L5. The only way to achieve perfect black bars with 2160p remuxes is to use L5. I’m surprised no one really mentions this for CE-NO. It’s quite a problem that you can’t see and therefore can’t use the UI/OSD while watching a simple UHD-BluRay remux with DoVi.
I remember this being an issue in CE-NG also (though it works fine in the forks) but does the FEL TEST file (as seen here and here) still freeze after “Level 2 *this will work only on CMV2.9 hardware” on CE-NO for anyone else?
Try out the latest nightly build. If the problem occurs again, it would be helpful for the devs to have logs. There’s no point in just posting here without logs.