Found it: If the progress.owned property is set to true (false is the default), minimizing to the jobs bar doesn't work. Would be helpful to add that to the documentation.
It seems there are still a lot of mysteries surrounding the progress indicators (some of which steje also pointed out): Different behaviour for internal/external commands, status not updating, percentages appearing without configuration...
My code is currently not fit for posting here, I'm afraid.
Maybe a detailed Example Script would help?
MartO