Currently Unavailable Usernames: If you are waiting for a specific username to become available, you can register to receive a notification when it is purged by viewing the user's profile page and clicking either the icon or the appropriate text link. You can view their profile page at http://exampleusername.livejournal.com/profile. (Replace exampleusername with the username you want.)
which is all very well if the account in question has not yet been deleted. If the username has been deleted and purged, there's an obvious link to follow, too.
If the username you want has been deleted, but not yet purged, you can access the Message Settings for the username by visiting http://www.livejournal.com/manage/subscriptions/user.bml?journal=exampleusername (Replace exampleusername with the username that you want).
I suggest adding:
If the account has been deleted but not yet purged, there will be a link on its profile page to the Message Settings for the account.
In lieu of a formal interface (as suggested in http://community.livejournal.com/suggestions/826254.html), this may make it easier for users to track such things.
My first userdocs post, please be gentle :)
(Edited to correct some wording)
(Edited again to account for reading fail)