How to repair external hard drive that fails diskutil with 'Disk Full Error'
Perhaps Disk Warrior will be of some help. It's the best for recoverying the volume:
http://www.alsoft.com/DiskWarrior/
Stellar Phoenix Macintosh - Mac data recovery software, recovers data from damaged, deleted, or corrupted volumes and even from initialized disks. 
They have a trial version, so I guess you can see if your data can be recovered...
Data Rescue II is the best data recovery software on the market for recovering files from a problem hard drive. Data Rescue II works when other tools fail. Data Rescue II is also completely safe to use since it does not attempt any risky repairs to the drive while its scanning.
FileSalvage is an extremely powerful Macintosh application for exploring and recovering deleted files from a drive or volume.  FileSalvage is designed to restore files that have:


been accidentally deleted.
become unreadable due to media faults.
been stored on a drive before it was re-initialized/formatted.



Entering:


  mac data recovery


into Google will list some firms who specialize in recovering data. Data recovery isn't cheap.