Sorry for this late answer (hadn't turned on notifications).
I have checked it out, and yes you are right. I didn't read the documentation, but used trial and error instead to fix my issue. Looks like both methods do work, but the way you're describing it is the official way.
Anyway, the bug is still present in the current version: I filled out the proper column names, but the join columns names are still not exported (but the original attribute's name instead).
By the way, the reason I need it is because I have to create a mapping to an existing database (which I may not change because of applications depending on it).
I would like to see this fixed in an upcoming release (I missed the update of today, sadly). ;-)
Keep up the good work.