external logoff page for Exchange iApp

We are using the Exchange iApp and APM in front of Exchange/OWA. We are using an external logon page to customize its appearance. But when the user logs off or receives a logon error, he/she still gets to see a page created by the loadbalancer.

 

I tried redirecting /owa/logoff.owa using an iRule back to the external logon page, but for some reason this iRule didn't get triggered. Does anyone have a working config for an external logoff/error page?