borked

Your crux is user bandwidth monitoring - good luck!!!
I don't think it's possible, personally - to monitor bandwidth, you need to check in/out packets at the interface, and will not tell you what a single user is using, unless you assign dummy interaces for each user (not recommended).

Alternatively, to use an Apache module, but this will only monitor an IP. Again, not good. A custom hack of the apache module may be possible, since you can set apache variables based on user, but I have no idea how to hack a module.

You could do it another, less accurate way of ignoring all html bandwidth, and just monitor what they are downloading - and assuming that each download goes to completion... dodgy

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For coding work - hit me up on andy // borkedcoder // com
(consider figuring out the email as test #1)