I went and experimented for you. When you set a variable, for the value, start typing "xbox" and you should see the XboxLive object in the auto-complete list. Then right after "XboxLive" type "." and you see a list of properties, like GamerTag. I hope this helps you.
FYI, I'm not sure what you're referring to when you say "little gray bar/button in the upper right".
Hey I have a question for you.
Is it possible to load mutable profiles in C2? say a local multiplayer game, and I want to load mutable gamertags and save data. (customization for example). I could for instance ask a user to sign in every time "A" is pressed on a new controller. But the Xbox object only says things like "sign the user in". It does not seem to offer parameters. and my worry is that player 2 signing in might sign player 1 out.
I could remember P1's gamertag in C2 in a variable. But If P1 was signed out then I could not write to P1's profile. (new customization or high scores for example)