> A big problem with recent versions of Thumbs Plus has been that the metadata handling is completely changed, and not well explained.
Yep :)
> If I use the File / Edit Metadata dialog to append keywords to a jpg image, they're stored just in the Thumbs Plus database, not in the .jpg file.
I can't confirm that and it's not how it should work. It *should* store the keywords in the file's IPTC header (and does, for me, but maybe not consistently.. I'd have to look deeper into that). And from there, it should only copy them into the database if you have the relevant setting activated under Options | Keywords.
Personally I don't feel very comfortable using the new metadata batch editor (because it rewrites precious image files and I don't think it has seen much testing) and I think it requires more testing, documenting, and possibly an interface overhaul.
At any rate, the regular keyword editor in ThumbsPlus (Assign Keywords [Ctrl-K] or through File Properties) deals only with database keywords, which have been in ThumbsPlus since long before the introduction of the first commercial digital camera.
The only mechanism in TP9 to transfer database keywords to the files themselves is through (Batch) Edit Metadata. In TP7 you would use the IPTC editor to perform that task and it did a great and reliable job - like you, I'm sorry that it is no longer available in TP9.