I found the following command to work on salvaging a corrupted dmg image. The solution started from reading your problem. I looked for a solution for three days.
hdiutil convert imac.dmg -format UDTO -o output.img
Background: I had used diskutil on OSX 10.8.2 to create an image of my iMac's internal drive. I transferred it up to my server and when I went to restore it to another disk it would not mount saying there were no valid file systems. I was devastated as I had too much confidence in the image and got rid of my timemachine images to save space. Okay I am a moron who should know better!
I googled the error and found this discussion. I tried all of the items and was not able to mount the dmg file. I knew the the image was basically good by running
hdiutil imageinfo imac.dmg which gave promising results:
Format Description: UDIF read-only compressed (zlib) Class Name: CUDIFDiskImage Checksum Type: CRC32
From a link provided in this thread http://discussions.apple.com/thread.jspa?threadID=1218394
I found a reference to http://vu1tur.eu.org/tools/ for dmg2img which DMG2IMG is an Apple's compressed dmg to standard (hfsplus) image disk file convert tool. I went to the read me file for the tool which had this note:
1. An equivalent command under Mac OS X would be: hdiutil convert <input.dmg> -format UDTO -o <output.img>
And that turned out to be the solution. I was able to mount the output file, which was the right size, 200GB, and extract the files I needed!
SUCCESS!