Yes this functionality can also be configured using scripts and the Execute plugin, but doing it using a plugin just feels cleaner to me

I've only created the web UI for configuration though, so it must be configured through that (or by hand). After configuration, remember to press Apply before pressing OK, otherwise settings may not be saved.
Also, you might need to refresh the web UI before the preferences section appears. It will look like this - http://twitpic.com/32ksj6
Egg: http://github.com/downloads/slai/deluge ... -py2.6.egg
(download this one if you're unsure, and place it in ~/.config/deluge/plugins.)
Source: http://github.com/slai/deluge-copycompleted