My rules are always applied after the message arrives if that matters.

I've tried various combinations of rules Specifing a meeting a or
appointment with my name and the name of the person who delegated to me in
the To line or their name in To and mine in CC or the reverse.  It appears
that when you are a delegate that receives meeting requests, your name
appears in the the TO line, not the actual recipient.  

I've not been able to view the headers on an appointment as I can for a
regular message, so I don't know if there is something I could match to in
the headers.