Returning values from InputFormat via the Hadoop Configuration object
Using the configuration is a perfectly suitable solution (admittedly for a problem I'm not sure I understand), but once the job has actually been submitted to the Job tracker, you will not be able to amend this value (client side or task side) and expect to see the change on the opposite side of the comms (setting configuration values in a map task for example will not be persisted to the other mappers, nor to the reducers, nor will be visible to the job tracker).
So to communicate information back from within getSplits back to your client polling loop (to see when the job has actually finished defining the input splits) is fine in your example.
What's your greater aim or use case for using this?