You cannot do that specifically with the 5508, but you could use a few work around's. First you could configure the time out on the SSID to force Web Auth to kick a user and then re-auth. You could also create guest accounts that time bomb after X hours. As far as rate limiting you could just use normal QOS policy on your switch/router/firewall to restrict guest bandwidth.
the solution is to be a simple passthrough where users agree to some terms and conditions and are then limited to several hours and capped at 100mb of data. I dont want to create guest logins (potentially over 1000 users) and Im quite happy to allow them to have full bandwidth, just capped at 100mb.
if I have to add another box can someone provide some examples? ie amigopod perhaps?