"I confirm the ©dir tag field doesn't help with Plex...
The mappinig options won't help you either with this.
But reading the Plex forum the solution seems to be the iTunMOVI tag"
Thanks for your continued support! That reference is one I found already, but I missed the part about director(s). Nevertheless, I'm way in over my head when it comes to making sense of how to use that. I'm not a stranger to HTML/XML but I can't see how it relates to mp3tag/Plex. I'm hoping there's a less abstruse solution but I'm beginning to fear that my stopgap measure of including Director(s) in the description is going to remain my only viable option for some time to come.