Hi again
Just an aside : I was rechecking what I've done about 4 or 5 months ago, cause as I said, I've been in drupal, and now it's a little puzzling to remember zencart way again, but I quite made the export part, and I didn't remember.
Works as the Music products does, so if bookx is active (by the user), products type 6 are exported and adds the bookx fields. For now i have

v_bookx_genre | v_bookx_isbn | v_bookx_size v_bookx_volume v_bookx_pages v_bookx_publishing_date v_bookx_publisher v_bookx_series v_bookx_binding v_bookx_printing v_bookx_condition v_bookx_author

successfully exported (as you said, the easy part)
Now, EP, uses the model as unique. In my case I use isbn as model too, but I guess it makes no difference
I've check the categories and it's more or less like genres. A categories_description and categories tables. If EP handles this quite well, it must handle the genres too.
So I give it a go.
And meanwhile, work with that featured that you mentioned.

Thanks
Best Regards