Hmm, maybe I’m getting you completely wrong, but if an actor is not yet spawned - how should he be able to subscribe to the broadcast?
The broadcast is fired, and only those, who are already subscribed (means they must exist already) can receive this one.
While any actor joining at a later time can subscribe for new broadcasts fired, but broadcasts from the past cannot be received.
So you can only check for some expected number of actors and wait, until they are all present. Or you fire the broadcast over and over, until the last one is joined - but in that case, the same question comes up: when to end firing broadcasts - in any case, you need to know, when the last actor joined, which requires to know, how many of them you expect to appear.
Not sure what you want to achieve, but maybe you can just set some flag for example in gamestate?/gamemode? - newly spawned actors can read this and perform some activity based on that information.