I am doing something similar, building Win8 Metro apps with Kinect. IE10 uses WebSockets (at least for now; I don't know if that will always be the case) - so connecting to your Kinect device via service may be the only way for now. This doesn't really help your 'no installation required' portion of your question, but it works fairly well, as much as I can tell.
Re: Gamepad API, I still think you'll need some intermediary to translate Kinect data into something the Gamepad API can use, as the Kinect doesn't show up as a gamepad. I'm not terribly familiar with GPAPI, but I doubt seriously if a non-gamepad device would be supported by that API.
Of course, if you get a community together of like-minded devs who can all standardize on a single Kinect-to-gamepad translator, then at least your users only have to install one thing to enjoy a library of web games.
Sounds like an interesting side project :)