Hi @krisknap - it’s a good question. The answer is: you can’t target an existing EMR cluster for your job currently within EmrEtlRunner.
However, we will be replacing EmrEtlRunner in time with a new Golang-based app, called Dataflow Runner (per this RFC), and that app will support targeting an existing cluster. You can see a sneak peak of the functionality of Dataflow Runner (WIP) in its README.