No subject

  bzr fast-export $br | bzr fast-import-filter -i sub/dir/ | bzr fast-import

does. That is, split the subtree in an *independent* branch. I used this 
myself and I see many use cases where this would be useful. When are the 
current actions of split and join useful?

As 2.0 is approaching, it would be a good occasion to change the semantics 
of these commands and move the old semantics to a separate plugin.

-- 
Gioele Barabucci <gioele at svario.it>