I’ve seen at least one extension: Introducing Seamate, an easy-to-use Seahub customizer
You may want to get in touch with the author: https://forum.seafile.com/u/D_FR
From that post it seems that the extension has to be reloaded on every seafile update, as seafile doesn’t have a built-in extension system, so my guess is that effectively all extensions are temporary patches.
Good luck, and if you get something working please share it here with the rest of us.