For those who are interested I have posted in the Wiki examples of using master users in very interesting ways.

In these example we will create 3 kinds of master users. The first will be users who can read all email for all domains. The next example will be users who can read all email for their domain only. The third example will be users who can read email of domains listed in a separate ownership table.

http://wiki2.dovecot.org/Authentication/MasterUsers

Go to Advanced SQL Examples at bottom

Let me know what you think. In order to actually make this work you'll have to apply this patch Timo just posted:

http://hg.dovecot.org/dovecot-2.0/rev/3ba227176cde