It seems therefore that the only change needed is to add the encoding labels above to the list that koha presents in the Z39.50 target edit page. Or am I wrong, have I missed something?
You're correct. But since this list is so long, and the probability to get biblio records using the vast majority of those encodings is near zero, the question is: which of those encodings do you need which is missing in Koha?
Is the current list of koha-supported encodings kept in the koha database, or is it hard-coded in the koha code?
It's hardcoded in admin/z3950servers.tt Koha page template. You can ask for an enhancement on: http://bugs.koha-community.org and propose a patch, or find someone proposing a patch for you. Kind regards, -- Frédéric DEMIANS http://www.tamil.fr/u/fdemians.html