I have added support for "Mention" "tag"s in my little ActivityPub/NNTP-server, so now I just need to actually follow the convention of setting such a tag when replying.
How to do that? A message hook to the rescue:
(defun illuminant-follow-up-keywords () "Add the From: user id to Keywords:" (let ((group (or gnus-newsgroup-name ""))) (when (and (string-match "nntp.illuminant" group) message-reply-headers) (let ((from (mail-header-from message-reply-headers))) (when from (let ((userid (cdr (gnus-extract-address-components from)))) (when userid (message-add-header (format "Keywords: @%s" (car userid)))))))))) (add-hook 'message-setup-hook 'illuminant-follow-up-keywords)
If the newsgroup is from illuminant, it is a follow up, and there is a fediverse user name to extract in From:, add a Keywords: header with that user id, for a mention.