Thanks to all. Have booked a night at the Souq Waqif courtesy of +Qatar and so far (+48 hours) there's been no email telling me it's no go because of not meeting the 12 hour restriction. So it all looks good atm.
One more question, -is it quicker to go straight through immigration on arrival, ie not as a transit passenger? There is an argument for parking my carry-on in the lounge and then escaping via the 'backwards' route, as others have done, but it does seem a little convoluted and don't wish to spend anymore time in the terminal than is necessary.