RADIUS not good enough?

If you have an SNMP server, there's "snmp-server enable traps deauthenticate".

If you are on the technical side of things you could modify the piece of code that I wrote for a WLC to create guest accounts.  I am currently working on logging of the users that are created with this code.  Then you could simply add up the users and and have date and times.  Find the code here: https://sourceforge.net/projects/simple-swag/   The original intention of the code was a simple way for administrators to provide simple Lobby Ambassidor like function to a simple web interface and then provide customized guest user instruction page.  In the background it uses ssh to talk to the controller and setup the account. Its written in PHP so feel free to try your hand at it.

if you have the wlc it is easy to get the active users.  i have

an external authentication page that gathers information and sets it in a database guest 'log' file.

i miss the session end point as there is no event that occurs, necessarily, when a user goes away.

i also use this to check where the user is coming in from. we have many locations (clinics and hospitals, etc) and we offer guest wireless only when the facility is open otherwise the login won't be allowed.

alternatively turn on RADIUS AAA -- accounting.  I'm going to check this next but it looks like it will create entries for start and end.