I'd like to customize a LogEventResponse for a specific event. But it seems like there is only one place to edit code for LogEventResponse unlike logEventRequest which has as many as the event you have.
You cant edit the response for a specific Logevent request. Instead you would use setScriptData() as Padraig showed above. This will set the response for the event inside the script.
The default LogEventResponse script will allow you to set some custom response code for *all* log events. In this case it would add to the response data rather than overwriting it.
Just so I understand your question, are you referring to fields that are returned after firing a LogEventRequest in the response?
Setting data to be returned is a simple as writing Spark.setScriptData("data", data); in the Cloud Code of your event.