If you happen to have a large collection of images, tagged with artists, from another site, then it could be possible to search for the image on e621 and potentially apply the artist tag. The same holds for character tags, assuming artists and character owners post images they have made or comissioned on these sites. This fails if other people start uploading the same images, such as a specialized fetish-reposting account.
This repository contains some of the tools needed to do this matching on Furaffinities site.
character_artist_tags_by_post.json
is a file produced by the sql
select post_id, tag_name, tag_type from (
select unnest(tags) as tag_name, post_id, row_number() over (partition by post_id order by change_seq desc)