only the top players and the reserves will be able to rep singapore (as national players) in 2015. If so, the Team of 13 are not national players but rather national player candidates. Once the actual players are selected, the rest become redundant. At most they can be part of a group of sparers to train with the actual national team, but they will not have the chance to be part of the 2015 national team again. Based on the reality that normal players also have at least career and family committments, it will be impractical to have numerous selection games in a year.
Technically speaking this selection method will be equivalent to a national game group A tourney whereby only players minimally Rated Class 1 (RC1) or who has certain rating points are allowed to play in.
If this was the case, then at the start whether the candidates are the NSA's members is not important. But once they are selected as national Team Playes, they do need to be members as both parties will be in a contractual relationship whereby both sides interests will be well looked after. If players are not members, then it will be hard for the NSA to justifying the deployment of resources to assist them.
One thing though, the NSA will need to ensure especially during the later stage of the national team appointment period, the players are still members as valid period of national team status and NSA's membership can be very different