That's a great suggestion, and something that I'd looked into at one point. The problem is that CopSpot only allows for HTTP traffic and not HTTPS. That will certainly be a big problem for a lot of my users. If there was an easy way around that, I'd probably try it out.
Alternativley you could install the copspot plugin on ipcop ( http://www.ban-solms.de/t/IPCop-copspot.html )
It implements chillispot and gives you a captive portal which can talk to you radius for AAA.
Kind regards
Yves