I think it's important to list as a coder releasing a hack, so I did from the first day
It's possible I'll let myself be persuaded to implement it in v1.3+, if lots of people ask for it. This as long as it doesn't interfere at all with the working of the slim-line part of it, of course.
Letting users string accounts together I know some admins will not want to allow for. I'm one of them, I rather link accounts for members so I'm aware of their aliases without checking an overview page.
As it stands I don't think making the option available in v1.3+ would be much of a problem, but I remain reserved on this.
The way I would implement it, if it comes to that will be something along these lines:
1) User A in his UserCP selects the option (if given permission based on usergroup) to link accounts.
2) User A tells the hack he'd like to see User B and C become a slave account, where being logged in on either will show the other accounts.
3) Users B & C both get a PM from the hack with a link to verify this. An acknowledge and deny link.
3.1) The acknowledge link would string the two accounts together, optionally sending a PM to the admin.
3.2) The deny link would strike User A (like the login strike system). On a 2nd try it would revoke the right to string accounts together, as well as PM the admin.
(saying they accidentally clicked Deny while they were in fact User B as well, this would allow them to try again, once.)
Meanwhile the admin, from the ACP section of the hack could bring up a report on linked accounts. Showing all master accounts with slaves indented (like subfora).