Yes it seems calibre can modify tags "on the fly" for SD cards. I find this
great!!. This is what I wrote in my last paragraph above. I did it, just trying my luck with the 2 Go SD card of my PRS-505 which contains 80% of my ebooks.
From within calibre (card and reader recognized), I opened my SD card library and just added tags (
and only tags) for a lot of books. It worked!! If this lasts (and it looks like), this is the missing feature I had been looking for a long time.
I tried to do it also for the so-called "main memory" of my PRS-505, but quickly ran into small problems (probably a kind of mild database corruption which later prevented syncing) and had rather restore my files from a backup.
With a terminal, you also can use the calibre tool called
ebook-meta to modify metadata. (see
man ebook-meta)
For PDF files though, calibre does not implement the "subject" field you can find in the metadata of many PDF files. So when he finds one of these "subject" fields, he just treat it like anothe tag and add it to its "tag" field (at least with a two words subject).
This is not very convenient because I did not find how to get rid of these added "subject" tags using the command line. This -for me- is a faulty behaviour which could induce later some database problems if you are not aware of it.