As a Star Alliance Gold I routinely do this when travelling on SQ and nine times out of ten I'm successful providing economy is not fully booked.
Mind you I always book an aisle seat in the centre row on the 747 or 777 flts and that always seems to increase the chances. From my experience the check-in counter doesn't have the authorisation to block seats and they politely ask me to go across to the ticketing agent counter where the SQ rep is and it's they that have the authority.
There are no rules about asking for such. Anyone can ask. But there is no guarantee your request will be granted.
It all comes down to how full the flight is and how helpful the check-in agent is wanting to be. You are unlikely to have a desirable seat blocked out, just the undesirable middle seats. Its most unlikely to happen on a domestic flight where QuickCheck is in operation. And its most unlikely to be done at any time other than check-in.
And the seats are never completely blocked out. They just to the end of the allocation list.