We have just asked our developers to branch their code before updating it.
When setting up a review for a branch, which needed to include some files which had not been modified in the branch, it was not possible to pick up those files in the review.
Since the branch was made by doing svn copy <Trunk URL> <Branch URL>
then making the required changes in the branch, it is not possible to capture any files which are unmodified in the branch, as before they existed, the directory path did not exist.
The ability to specify a tag revision might help with this - not sure how subversion diff would cope though.