Non answer here, but I cant add a comment yet so adding this as an [non] answer.
For Chrome, it seems to run "xdg-open" when a magnet link is clicked with the magnet URL passed in as the $1 to the command. This does successfully trigger Deluge to open, but it fails to actually add the torrent.
It used to work just fine for me in 14.04 and I just recently took the dive and upgraded to 14.04 -> 14.10 -> 15.04 and now it's broken. So perhaps seeing that xdg-open is a rather simple wrapper to call Deluge, perhaps something changed with how deluge wants the magnet link passed to it from xdg-open?
Update: Looks like the culprit could be with gvfs-open, which is what apparently xdg-open calls. Looks like 1.22 worked, 1.24 doesn't.