I'm planning on trying this when I transit DOH in September - I appreciate all the details the OP (and now others) have provided, but I still have two questions
What did you say to the security guards when you went backwards through Transit security? Was this the part of "explain that you want to go through to arrivals" or did people use other wording?
How big are the storage lockers in the Al Mourjan lounge? I have a Pelican case (which is smaller than my standard rollerboard) and a messenger type bag. I assume both of those will fit?
thanks
tom