I am just back from a trip in Moscow. I found out just by chance that at DME there is an immigration line, the very last on the left, that says "Diplomats" but in reality they allow all passenger with a Business or First class incoming boarding pass.
I tried myself with a SQ F boarding pass, it went smooth, no question asked.
As it is not officially advertised, they may remove it at any time, but so far it works