@teegee543: I guess it should be available as metadata somewhere in the MOBI/EXTH headers. Is there anything about that in the Wiki?
EDIT: Yep, apparently exth record 113
It's stored in the json file as
Where asin is the ASIN, and TYPE the doctype (exth 501), most of the time that'll be EBOK
(#B012345678^EBOK for example)
instead of the usual