I'm currently working with a customer who had a requirement to backup the mirror database. You may wonder why they want to do this - there are a number of reasons which I won't go into right now. The simple answer is that it is not possible to backup the mirror database as it is in a recovering state. It is not even possible to use any SAN-based solution to take a clone/snap due to the same reasons. What I mean by this is that whilst it may be possible, it will not be supported and the database consistency cannot be guaranteed.
Maybe a feature request for Katmai+1?