Hi Valter,
I really don't think that this is going to be possible in a supported way due to the nature of the Outlook client - the first thing it will do is to check the user's credentials using the database - and if that doesn't happen, no other customisations would function.
You might be able to create a completely separate Outlook Add in that checks that CRM is operational using the SDK webservices, and if not, display a splash screen.
hth,
Scott