Elisa, thank you for your notes. Really helpful.
In my case, I'm less interested in using plugins for listening enjoyment (EQ, saturation, etc.) and more as a vehicle for using MusicBee as a quick test tool when I don't want to fire up a DAW to check various aspects of audio using metering, room correction, track coherence, etc.
So now I've noticed another issue. Per your thoughts, I've created a unique MusicBee VST folder, set the MusicBee VST plugin to use that folder, and copied one core monitoring plugin, Sonarworks Reference 4, into the folder.
MusicBee successfully scans the Sonarworks plugin and lists it under "Equaliser and DSP Settings", which is good. I can activate the Sonarworks plugin, launch the plugin interface and make edits. The problem is, anytime I make a change to the plugin, the change is not reflected in the audio output unless I uncheck (deactivate) and recheck (activate) the plugin again in the Equaliser and DSP Settings window. I can literally perform the most audio-mangling changes in the plugin interface, and nothing changes in the actual processing until I uncheck and recheck the plugin. Effectively, it seems like the MusicBee VST plugin just takes a single snapshot of the Sonarworks plugin settings at activation and uses those, regardless of changes, until the plugin is deactivated and reactivated.
I tried another plugin, DMG Audio's TrackControl, and making changes in its interface does nothing to the audio output whatsoever, even after deactivating and reactivating the plugin.
At this time, it appears the MusicBee VST plugin may be in a fairly broken state. It obviously works with your plugin selections, but it's looking like some of the basic plugins I'd want to use simply don't work through its hosting mechanism.