I fly this route a couple of times each year and most recently returned from SVO the last Friday in April (prior to the holidays). Both on this return and one this past fall, I was on a B fare and at the time of booking J9 was showing with Z0.
Also, I have only flown 763's on this route - no lie-flats yet.
In the fall, with three days to go and J6 showing, my Z upgrade loaded.
On this past trip, J dropped to 0 a few days after I booked and hovered between 2 and 5 on in the two weeks before departure. Most likely because of the holidays, it was not looking good so about ten days out I switched my return from SVO-JFK to SVO-PRG-JFK (in BE). Boy am I happy I did. I still was wait listed for SVO-JFK and since both SVO departures were within 15 minutes of each other, figured I could see what would happen the morning of departure and switch back if Z loaded.
Suffice it to say, that Z did not load and as a PM with but a few seats remaining, the GA noted that I was most likely not going to clear as I was too far down the list. So I kept my connection via PRG.