If the master table is kept (now the default) SQL Developer 4.1 (EA1 and above) can find the log file name.
Steps - launch export/import as usual then:
1/Go to dbaview -> Data Pump -> export (or import) jobs
2/Pick a job that is 'Not Running'
3/LOG FILES output shows the Log file from the server.
Log file viewer contains a few extra lines at the top.
State (schema/job id) is not cleared if you change connection.