If you use HLTV event as new round detection, then only surviving players are alive.
Players are spawned the same frame, but after this event.
Also, some players may be in Appearence Menu Selection and in that case they won't be respawned.
IsPlayer won't help on this i think because it always returning true on players.
If you can wait untill RoundStart, there you are granted that all players have been respawned.
If you really want to do this at new round, you can hook CHalfLifeMultiplay::RestartRound POST with orpheu.
If you don't want to use orpheu, you can register FM_StartFrame at HLTV event, and when it is triggered unregister it and execute your code (this method is used in invisibility armouries plugin by VEN).