With those clues, I've managed to get it to work (at least for a small sample size so far). For the benefit of other folks, this is what I've found from experimentation.
The rules are:
Artist picture filenames must start with the artist's name, and match the artist's name in the ID3 tag.
To have more than one picture, of course, the filename must include a numeric counter. 2-digit and 3-digit counters work (didn't experiment with others).
Spaces are OK in the artist name, and between the artist's name and the counter (underscores work but aren't required).
Other information in the filename (date, location) are not allowed before or after the artist name.
Because other locations aren't allowed, I didn't experiment with redirecting the folder. I'm going to have duplicate photos anyhow, because I must keep my originals with date and location in the filename. I was hoping to avoid duplication, but that's not possible with the current version. Also, artist picture collections will be more labor-intensive to sync, as I accidentally add new photos to one folder but not another. But... the final product is still great.
Thanks, guys.