I have noticed that the limitation of the APs, even with open source firmware is that they dont have enough resources to run perl or Mysql. The trick is to run a slim version of a captive portal. Example : NoCatAuth type apps. If you can get an AP that you can add storage to , like the netgear usb versions that you can flash maybe. The other option is to use Alix boards connected to a regualr AP will work wonderfully :) Hope this helps :)
DM