[vouwfietsman]

> But dynamically updating the set of components for an entity in order to "send it" to some other system, and relying on the ECS as a query database to coordinate all this, seems like questionable practice to me.

Indeed it is. The idea of components is to decouple systems. Where systems generally operate on few components, and own a disjoint set of components. It is not a communication channel, just like any other database should not (primarily) be used as a communication channel.

You can combine an ECS with any number of different patterns to make your communication work: events, agents, messagebus, you name it.