Good news for MSN users!
Thanks to the advice of a Microsoft developer, I even fixed the legal issue that prevented us to enable this by default. Client applications (as opposed to web applications) are not meant to ship the app secret key, even for closed source app (would be too easy to find the key in the binary). So there is a simple checkbox in the app settings to tell it is a “mobile application” and then it won’t need the secret key to authenticate.
All patches got merged and released into empathy (>=3.3.2), telepathy-gabble (>= 0.15.0), and gnome-online-accounts(>=3.3.0).