You DO know that there is already a webcam available for the Xbox?
From Wikipedia:
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Мicrosoft has also revealed an Xbox Live webcam, for video chat. The webcam has been exclusively released in Japan, due to high Internet speeds and it carries a price tag of 6800 yen (roughly $60). (Official Xbox Magazine — March 2005 — Pg. 26)
I hope you have experience with programming drivers, especially since the XDK does not have much support for (development of) third-party hardware drivers.
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Does it waste lots of RAM / CPU time to handle all USB stuff twice?
the host controller handles all of the usb stuff - no programming required - the drivers basically just point the host controller to where the data is. So , no, it doesn't WASTE any more ram/cpu time than would normally be required.
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Is it possible to make the XDK never touch anything USB at all, by for example not calling XInitDevices()?
The usb stack isnt created until xinitdevices() is called, so to answer your question...yes. The drivers i've written never call xinitdevices(), i just create my own stack similar to how xinitdevices() would.
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An "XDK USB driver replacement" could be very useful for the Xbox homebrew software...
This indeed would be useful. I think the best way for this to work is to just allow the usb driver to load external drivers, then you can just write drivers for your individual program needs. You wouldn't want to create the usb driver with a limited number of included drivers, because then it would be the same as it is now - not fulfilling everyone's needs. I will look into this subject in the near future.