Definitely possible at check-in (that's what I do) and I'm guessing checking in early online might help? I've never used SQ online check-in though.

Don't know how different airlines work, but with some you might get locked into a particular seat once you make a generic seat request. That is what always seems to happen to me on JL - you can call ahead for a window or aisle but not a specific seat, and they can immediately assign the exact seat over the phone but once this happens the seats cannot be changed, even at the airport.