Ok, so you said that you have a few computers sitting around that you can throw at this? Ok, I have two options for ya. First, try PFSense. It has a pactive portal option built in and you cna brand it however you like. You can even design your own Captive Portal page. It also works with RADIUS. Second, you could look at Untangle. It also has Captive Portal but is not as configurable. If you want Untangle's captive portal page branded, you'll have to pay for it. Both of these are linux/FreeBSD firewall/UTM distributions. PFSense starts up a little faster, but Untangle is a lot easier to configure.
