I've not tried it with HFS filesystems, but when I accidentally did the same thing and dd'd the first few MBs of a drive once I successfully used
testdisk to scan and repair it with minimal loss of data (couple of small files were hosed IIRC, the rest was fine).
Naturally any attempts to repair the partition table can be potentially destructive so generally it's a good idea to create an image of the disc in question and perform the recovery operations on that if possible. If you can handle the time taken, disc space used, and the level of irony, dd the drive into an image file and point yer tools at that instead of the actual disc so you can test if it works first.