Add a check of the edit summary, for any links to a username. Don't require the ~~~~ check in these cases.
Eg. [http://en.wikipedia.org/w/index.php?title=Wikipedia_talk:Signatures&diff=prev&oldid=556815553 this edit],
Eg.2. [http://en.wikipedia.org/w/index.php?title=User_talk:Peteforsyth&diff=prev&oldid=554514702 another edit]
This would allow us to "ping" a user, in any namespace.
It would allow us to notify someone that their question had been answered, or query something about their edit, without working their name into the text of the response.
My only concern is that this might get over-used, leading to some power-users "pinging" too often/easily. Ie. trying to use edit summaries as a communication tool. Eg. "Does this look right [[User:Ironholds]]?"
Possibly the benefits outweigh the potential drawbacks?
(I thought Notifications was already doing this, hence I've been linking usernames in some edit summaries for the last few weeks.. >.< )