DiskWarrior will still work even if you don't have the contiguous disk space. Just don't pull the plug on your Mac while it's working.
Basically, what it does if you have enough contiguous space is to write the new directory to disk before deleting the old one, so that if something goes wrong while it's writing, no damage is done and it can just use the old directory.
Without the contiguous space it just does what all the rest of the disk utilities would do anyway. As long as your power doesn't go out while it's working (which is a pretty low chance) you will be fine.