> Is there a plan to add a config for a listener to a non-service task?
No, altough you can have regular execution listeners.
> I am not sure if there is a resultVariable concept as it is for a serviceTask.
No, only for the service and script task. Where would it makes sense otherwise
> I want to set it in the process variables using something similar to activiti:resultVariable.
You can do whatever you like in the listener, including setting a variable.