I think it could be useful to have plug-ins in the Captive portal.
I can see at least 3 places:
1) before the page asking for credentials;
2) after the credentials have been validated;
3) in the pop-up window.
1) before the page asking for credentials: I believe that in many organization, you have to authenticate to some sort of server when you launch your PC. Then you have to authenticate to read your email (local email server), then you have to authenticate to ZeroShell to access Internet. The Captive Portal could access tiers that could grant that a user has already authenticated on that machine and that another authentication is superfluous.
2) after the credentials have been validated: some users tend to ignore email, messages, etc. some will get over quota and just do nothing about it. After they have authenticated to ZeroShell, a third party could be allowed to deny them Internet access based on such or such ground.
3) in the pop-up window: could display data like number of unread emails, quota, etc.