Coming back to this now that holiday hysteria is over....
Thanks Chaley. I set this up at the time you posted using option number 1 (because I thought I understood it and using it saved me investigating all about plug-ins) and it seemed to work as I had hoped.
However, on opening calibre now, in my new Virtual Libraries column most entries read "TEMPLATE ERROR maximum recursion depth exceeded" but two read "TEMPLATE ERROR maximum recursion depth exceeded in _instancecheck_". Outside of those, one title is correctly labeled in the column but doesn't appear in the "VL not yet assigned" virtual library, and one title shows up in the "VL not yet assigned" virtual library but the field is blank in the Virtual Libraries column.
(Note to TheDucks: Your message was too cryptic for me to decode.

)
A search on the error message shows nothing in the forum.
If there is a quick fix, I'll stick with option 1. Otherwise, I'll undo what I've done and experiment with the plug-ins.
Any advice?
TIA.