If you can find an event that is triggered after the user is logged out you could redirect then.
You could add a urlMapping to web.config for "util/logout.aspx" and do a redirect from a custom "logout.aspx" to your ISA logout-url.
That worked like a charm.
Is there a way to change the logout button in edit/admin mode, what I want it to do is to redirect to an external page.
The reason for this is that we uses SSO with an ISA-server and the ISA-server handles the logout.