Greetings!

Hmm... I thought about this kind of thing before for a few different projects... Random ideas:

a) Are you putting a machine in the room, or just providing the network link?
b) If you have a machine in the room, you could fashion it into a "kiosk" type situation. There is software out there, I can find out more if you are interested, that limits the browser to a specific group of sites, locks down the OS, that kind of thing.
c) If the hotel uses magnetic cards as keys, you might be able to rig a card reader into the system to "unlock" the kiosk, keeping out unauthorized access and starting the billing. You could have the billing autoterminate with an inactivity timer, say 15 minutes of no usage.
d) Billing by bandwidth can also be done by measuring packets from the interface.
e) Another possibility is to use a proxy server, and have that do your metrics / billing for you.
f) I would recommend that the billing not be a feature turned on or off by staff. Rather have it feed into the normal biller, just as telephone usage would.