Kafka from client to server vs. "router" in the DC
Kafka doesn't care how you use it; it does nothing except transporting your arbitrary messages for you. Yes, you can use it that way, and actually I do not see anything specific enough about it to make it necessary to be mentioned in the docs.

Obviously you need to take a  little care regarding security (SSL or SASL...) but aside from that, there should be no particular problem.