I had an 8 hour, 15 minute layover which qualified me for STPC. But my experience twice before showed me that by the time I get to the room I'll probably only have 4 hours or so to sleep. So I decided to ask if I could have the airport hotel instead. I first asked about 2 months in advance at my local QR office. But they said they couldn't confirm it for me. So I waited until the actual day of arrival. I presented my STPC voucher at the transfer desk and asked if I could get it changed to the airport hotel. The agent there, who was very helpful, tried calling around to the hotel and her supervisor. But in the end they said I couldn't have the airport hotel for STPC. So I 'settled' for one of the sleeping rooms in the Safwa Lounge.
So it's just one data point, and in the past some people may have been given the airport hotel for the paid stopover. But at this point it's looking extremely difficult to get it.