Played around with the gamepad plugin. It does not appear to work for the Xbox. I just changed a function that was activated on touch to an on 'X' pressed event, and it didn't work. It did work with a wireless 360 controller on my PC in Chrome.
What I did discover is that the D-Pad corresponds to the keyboard Arrow keys, so if your game uses keyboard events with the arrows, then you're set. The start button acts as a keyboard key, but I didn't test which one yet. I'm assuming maybe the Enter key? Otherwise, the 'B', 'Y', 'RB', and 'LB' are already being used as shortcut functions for operating the IE app, so those would be taken up even if the gamepad plugin worked.
I have a side question though. I noticed my HTML5 was creating saved data on the Xbox 360 storage, much like Chrome stores it as HTML5 appdata. Unfortunately deleting it didn't clear the cached website, so I always had to upload to a different directory for the browser to realize it was a new version. Is there a method to clear website data? You know if you want to update your HTML5 game on your website and keep it in the same location. At the moment it seems I have to manually delete any cookies or appdata in my browser for it to recognize it's updated.