PAYG Wifi Hotspot Using OS X Server and Airport

Has anyone ever setup a PAYG public wifi hotspot using Apple kit? I have been experimenting with Radius but it doesn't seem to be what I thought it was. I am looking to create the type of Wifi where users log into a web front end which authenticates against a remote server. Like BT Openzone for those familiar. How is this usually achieved? I am looking to set this up myself, not just buy a turnkey system from an existing vendor.