Public / Free Internet Access portal using external logon page

If you can wait a little bit until v12.0 is released this summer, you should be able to do it using IP-based sessions that will be available by using APM. You would essentially setup a transparent proxy with a captive portal. Not to say that you can't achieve what you're looking for today with some iRules, but I believe that the complexity and troubleshooting of doing it today vs what you'll gain with this capability in v12 is worth the wait...