Maybe a good chance they can also get together for a decent chunk of time in June before the playoff game. Europe based and A league players anyway. As those seasons will have finished.
Yes the guys who would not be available prior May 30th (start of the window), most likely only be M Boxall, Tuiloma, De Vries, Pijnaker, Rogerson & N Boxall. All in season with their clubs who play through NH summer. And the latter 2 probably won't be picked in a 23 man squad.
Costa Rica will have the same issue for any guys they have in the MLS.
Most Euro club seasons will finish early-mid May.
EPL finishes May 22nd. League One late April, League Two the first week of May. Singh's J Regensburg 15th May, Cacace's Empoli 21st May, Thomas's PSV 15th May, Champness at Giresunspor done by 21st May, Danish top flight for Bell 22nd May & 2nd flight (Just, Stamenic & McCowatt) right up to 28th May - but maybe HB Koge are already playing for nothing by then & Helsingor have promotion confirmed. For Marinovic his Ligat ha'Al season will be over 13th May. And so it goes on
So you can have a pretty full strength squad for say Friendly 1, late May. Then a totally full strength side for Friendly 2, about 4th-6th June. Then the Playoff 13th or 14th June.
Hay may even pick a 30 man squad to go into camp for the friendlies, especially if some guys are carrying injuries - before only finalising his 23 man squad after 2nd friendly. Some plan like that.