I was able to discover some files being written with Canon’s LensType field. I know this to be a generic lens in my case, not the actual lens used when taking the images (I’m using a lens adapter that necessarily lies to the camera.) And for now I’m using Apple’s Photos, which is ignoring the usual EXIF stream here and/or giving precedence to the Canon stream. So edits to EXIF, XMP etc have no functional effect in this case.
And so I added Canon:LensType to MetaImage’s range of options via the Preferences. Now when I open these images in MetaImage I can see the generic lens info revealed, and choose another one from the list OR paste from the clipboard (my preference, since the lens isn’t in the list).
I save the images and MetaImage does so. Clicking around within MetaImage it appears the edit was successful. Close out the selection and reopen them, and it’s revealed the save never occurred.
Mind you, I’m not even trying to edit via the Photos plugin at this point. I am just opening the images in MetaImage for editing, saving, and importation into Photos later.