I meant that this works with the property set in the subtree.
Ah, I understand now.
So add the EXPORT_FILE_NAME property to each subtree, and then do
C-c C-e C-s h h
*for each subtree of interest*.
Got it. So the EXPORT_FILE_NAME property is exclusive to subtree scope exporting. Even though a tag is present on only one subtree, exporting by selecting that tag will not do the same thing. (I assumed, hoped that would happen :))
You could probably org-map your way
through the subtrees and export each one by calling
(org-export-as 'html t)
N.B. the subtreep setting in this call.
org-map is a golden suggestion! I'll dive into that.
All said, looks like ox-publish misses an ":export_file_name" option. Don't you think?