--label switch will not be used by Clear Case in future anymore because they don't have any meaning for "diff". So You can probably even consult whole new parameter. Please track this discussion: http://goo.gl/6DUP5 because it maybe change whole idea how generate diffs.
SVN Changelist Support (Hack)
Review Request #2099 — Created Feb. 2, 2011 and discarded
This reuses the --label feature from ClearCase to offer a means to use a subversion local changelist name. It might be more correct to add a new switch (--changelist), but this works nicely for a small hack.
Verified that it works the same as before without --label, and that it only includes the appropriate files in the changelist with --label.