>"Force Close" OK but I'm not so sure about minimizing/hiding: I thought that this part was handled by the clients.
Compositor can hide/minimize windows forcibly (and restore them back if unresponsive application recovers). I.e. it might look like:
1) A window grays out.
2) "Force Close" and "Hide" buttons appear
3) You press "Hide" button and window disappears.
4) An icon in the system notification area appears notifying you that there are unresponsive applications. You can right-click on it and bring up a list of forcibly closed apps, with possibility to unhide or kill them.
5) If application recovers, then the user can be notified by flashing icon on the notification area.