svn up "dry-run"

I have a working copy with directories d1..dX in, and a repository to which
directories occasionally get added, say d1..dY where Y>X (though the
directory names are not numerical).

>From the working copy, is there any way of telling what directories are
missing on that working copy? I don't want to do an "svn up", because I
don't want to lose the current state of d1..dX. I can't do "svn up dY"
because I don't (yet) know the name(s) of the missing directory/directories
dY.

What would be perfect is "svn up --dry-run", producing identical output
to svn up, but not actually doing anything. I could then parse the output
of that. However, that command doesn't' actually exist.

I suppose I could parse the output of "svn diff -r HEAD" but that
seems pretty expensive given it's diffing millions of files and
I only really want the current directory (not subdirectories).

Any ideas?

-- 
Alex Bligh