Create a trigger that compares a field, with a field in other table
You can't create triggers on system objects. So going down the path you're thinking isn't possible.

Your best bet to increase fault tolerance is to move your Witness database to it's own server hardware, (not just another instance on the same box even though SQL Server allows you to set it up that way), separate from both the principal and the mirror.  

That way you're highly unlikely to have a failure that takes out both your principal AND your mirror database at the same time.