Just a side thought here.
This 'problem' could be solved more radical - and more logical.
If we presume that hotspot IP's, like the disconnect window, are always local LAN IP's, it would be nice to say to your browser "do not accept popups from the Internet…..' but popus coming from the LAN range should be accepted (always ?).
Popups from a local source can’t be really annoying, because YOU, or the one that gave you YOUR connection, controls them.
So: browsers should accept, or handle at a second level, popus coming from local IP’s (gateway in this case).
On the Firefox feature request list this item exists.
IE: keep on dreaming….
When the gateway isn’t local, the problem is solved otherwise: users are asked to keep the main login windows open, so they can disconnect (they have to open a second browser window!). This is important because many are actually billing for their ‘wifi/network’ usage. Loosing the disconnect windows/button introduces the opportunity for the operator to take another hour, or more, before disconnection for ‘non-usage’.