[vezuchyy]

You can always use an open network to generate passwords for the proper internet connected WPA-EAP network (along with some in-flight multimedia like some carriers do). Extra step for sure but it solves the problem.

PS: I'm a couch expert so I have no idea if there's a problem with this idea.

[Wowfunhappy]

The first problem that comes to my mind--clients will remember both wifi networks and may continue to choose the open network when e.g. waking from sleep.

The user can go in and forget the open network of course, but most won't know to do that.

[stereo]

It would theoretically be possible to bounce clients which you know are already authorised on the paid network.

[lxgr]

You could just prevent access to the wider internet on the open version!

[IggleSniggle]

Use Bluetooth instead of WiFi for the password generation channel