Did you ever get this to work? I'm working on a personal update python program (so no worries about credentials as it only ever has mine and runs on my gear)??? I'm still a bit lost with it.
First, i don't know how happy people will be if they have to type their username and password in a random page, so, keep that in mind. i would recommend to either do a plug-in like the tower ghost plug-in, or do something where you can take the user to a browser page and let Sony or Microsoft handle the login and just wait for the redirect to Bungie.net and get the token from there.