It probably needs something else beside the "Microsoft Account Sign-in Assistant" service running...I've enabled that one too, but am not able to add a username @outlook.com, since it cuts me off on the .com...maybe someone else can figure the rest of it.
On a client Windows 8 OS, is this service running permanantly?
Update: It seems that on Win 8, like in WS2012, this service is set to start manualy, meaning that it will only start if you log in on a Microsoft, not as a pre-requisite of it. I have enabled Microsoft Accounts in Group Policy, no use. My guess is that this feature is absent from WS2012.