Have discussed this previously with an SQ pilot friend of mine. Its at the captains discretion pre and post flight and absolutely not going to happen during the flight.

However pre-flight he explained they are usually occupied with loading, checklists, routing, ATC, weather etc and will usually reject any requests. Post flight is the best the opportunity. An hour or two before landing ask a member of the cabin crew to submit a request for you via the CSD. And apparently if the flight has an on time or early arrival its more likely to be agreed to