[...] Or is there a reasonably simple way to do it myself that I'm not seeing? Weird that Calibre can't do it, no?[/QUOTE]
mobi2mobi is a simple tool to do this very easily, see:
https://www.mobileread.com/forums/showthread.php?t=21763
actually calibre alo has a tool to edit metadata:
http://manual.calibre-ebook.com/cli/ebook-meta.html