I saw on the spec page that you were considering limiting the device element to audio/visual streams. I thinks this would be a mistake.
I'm currently working on integrating serial port connected hardware with a web application I'm developing. The only solution is for the user to install local adapter software. This defeats the purpose of using a web app over desktop software. In order for web applications to be gain traction over desktop software, they must be able to interface with usb/RS232. I believe the security trade off is worth it. I also believe that the working group should make the device element spec a high priority. If you don't google will probably implement their own version for chrome OS(it will be necessary in a browser based OS model). Thanks, Seth Brown