Is there a way to 'sparsify' a working copy, without starting over?

I have a (full depth infinity) working copy:

  parent
      \-- sub1
      \-- sub2

I'd like to make it sparse, throwing out sub2, so it becomes:

  parent (empty)
      \-- sub1 (infinity)

Is there a way to do that, without first making 'parent' entirely
empty (by executing 'svn up --set-depth empty parent')? Making
'parent' depth=empty would also remove sub1 which I then have to
retrieve from the server again.

Note: removing sub2 by running 'svn up --set-depth exclude
parent/sub2' is not the same (parent would still be depth=infinity, so
new subdirs could appear on subsequent updates).

-- 
Johan