I'm not going to make that change to calibre at large, but it is a trivial change to make, so if you're willing to run from source, I can tell you what the change should be: In the file metadata/mobi.py at line 364, add the following two lines:
Code:
if self.original_exth_records.get(501, None) == b'PDOC':
update_exth_record((501, b'EBOK'))
Also make sure you unset the pdoc setting in MOBI output preferences.