'I would suspect that many passengers would not be so keen on bagging such seats if this was enforced'
Rather depends how the passenger got to the emergencxy exit seat in the first place. If they got it for free, they may be put off by a 'no alcohol' policy. If however, they are tall (like myself) and pay the extra for a preferential seat (invariably exit row) on a long haul flight, there is a good chance they'd just put up with a 'no alcohol' policy. I know I would, if it meant reducing the chance of a seriously aching back after 12hrs.