I'm not that knowledgable on using mdadm, but wouldn't dd-ing the header of another raid drive just create a clone of an already existing RAID member since they have the same disk identifier?
Also, I'm doubt that RAID layouts adhere to the 512 bytes where the MBR is usually written.
Look
here for more information on the RAID superblock versions. It lists info on multiple versions of the linux kernel raid superblock (where the raid information is actually stored on disk)
According to
this, I believe mdadm uses the superblocks .90 or higher.
EDIT: Even if the ext filesystem didn't overwrite any data on the disk, I still don't know how you would go about recovering the superblock and restoring the correct UUID.
Try reading
this.
Pascal