Mike,
You're not the first to ask for this. However, it seems the serial number is either not encoded into the file at all, or at least encoded in such a way that it can not (yet) be deciphered (even the latest version of ExifTool isn't able to do this).
Perhaps in the future it is possible to decode the serial number. If this is the case, I'm sure Camerabits will add this to PM as well.
Cheers,
Hayo