I think I've finally got a decent technical understanding of how PGP interacts with the MBR, thanks to the link here (http://www.storagecraft.com/support/forum/missing-...).
Basically, Symantec uses up to the first 63 sectors (track 0) for their PGP MBR. The original MBR is backed up. When the disk is instrumented, the original MBR is restored. My question is where is the original MBR stored on the disk? Somewhere on the hidden track (read: track 0) or within one of the encrypted partitions? When you back up the MBR, I'm assuming it backs up the Master Partition Table and disk signature, in addition to the Master Boot Code (basically, all 512-bytes).
Also, when running the PGP WDE --recover command, I see it searches throughout the disk. Is it searching for a signature, at the low-level, or is it searching at the file-level? It looks like it's combing through sectors, so I'm thinking it's looking for a particular signature to attempt to piece together the PGP instrumentation of the disk.