In many cases we could detect the book format (Hardcover or Paperback) based on information given in the 020 (ISBN) field, e.g. these records in the kotona test set:

005083536 020   L $$a952-5329-15-1$$qnidottu
005083536 020   L $$a978-952-5329-15-5$$qnidottu
006458254 020   L $$a951-52-2096-3$$qSöderström, sidottu
006458254 020   L $$a91-7486-778-4$$qAtlantis, sidottu

If 020 $q contains nidottu -> Paperback
If 020 $q contains sidottu -> Hardcover

However, this heuristic is language-specific, so I'm not sure whether it's appropriate to put that in bf-to-schema.rq. OTOH, it's not going to hurt even if this kind of extraction fails...