> I've been looking into Kafka lately, because I have a need for "the thing that Kafka is in practice" (not a message queue per se; rather, an indefinitely-durable event store that supports message-queuing API semantics for consumers.)

Don't think that is a secret feature. It was an intended feature, from the author himself [1].

[1] https://www.confluent.io/blog/okay-store-data-apache-kafka/