Whew. Finally found the answer after Adobe support led me up all kinds of blind alleys (ultimately crashing a whole disk. Ouch).
I went searching in the SDK for XMP and thought that perhaps a metapanel template had gone wrong, although it would seem odd this would occur on three separate computers.
Yet that's what happened. On a Mac, if you go into /Library/Application Support/Adobe/XMP/Custom File Info Panels you'll find some panels. But there were four adrift, with .txt suffixes, which seemed odd. So I removed them, and BINGO! "additional metadata..." now works.
I have no idea how they got in there. On all computers. I noted that the text in one, "Camera1.txt," was listed in an Adobe blog way back in CS 3 talking about custom panels. Perhaps a plugin put them there, or whatever. I sure didn't do so specifically.
So if anyone ever runs into this problem, here's your solution. Cheers.