The over age players will probably end up being A-League guys cuz its in the middle of the MLS season and the UK pre-season. So I'm guessing Payne, Rufer and Kosta.
I think this is a likely scenario.
Though Smith maybe released by Colchester for a few weeks, July/August. He's done plenty of pre seasons, and knows what's required to prepare for another 46 game slog in League Two. He will also be conscious of the need to prove himself to Hay, with competition for the AWs backup CB roles to Reid/Boxall heating up.
Reid could be preparing for an EPL season with Brentford, back training with West Ham, or trialling with some other club in the UK! His situation is so uncertain that I think he'll give Tokyo a miss. Though had a look back and he wasn't playing at the London Olympics, when Smith was. So this will be his last shot at an Olympic experience.
Some of the U23 players will also be far from certain. Bell mid season in Norway, and Singh likely in the midst of pre season at a new club, in what is an important juncture in his club career. Woud & Almere maybe preparing for the Dutch top flight.
So yes expect many A League players (off season) to come into contention.
Smith, Payne, Barba, Rojas, Wood and Musa. Guys currently at pro clubs, who were all at London 9 years ago. Adam Thomas, Howieson & Weemac also still playing at a level.
https://us.soccerway.com/matches/2012/07/26/world/olympics/belarus-under-23/new-zealand-under-23/1281833/