Radio Sultanate of Oman in Muscat, Oman finally acknowledged my March 2011 reception report with an email, today, thanks to a tip from a fellow short-wave listener in Brazil.
The correspondence states: "Thank you for the email and for the nice report that you sent. I will try to send you the QSL card as soon as possible."
This has been a difficult station to get a verification from, but after four attempts it looks like this one can be added to the collection. Hopefully the QSL card will be forthcoming.