Triggering Session_OnEnd When User Close Window

We have a solution where we want to control the number of users logged
in at the same time.

So when the user log in we check number of currentusers agains
allowedusers and increment a currentusers (in the database).

On Session_onEnd or when a user logout (actually I abandon the session
on logout) we decrease currentusers.

But as we all know when the browser is closed or the user just go to
another site this decrease does not appear.

It will of course trigger when Session goes to timeout, but I imagine
a lot of angry users calling support when they are denied access.