Okay, I found out.
You just need to insert a few records into KWISClient.db. No BookCover.preference file needed.
INSERT INTO main.extra_identifiers( extra_identifier, poll_interval ) VALUES ('CuratorBookCovers', 0);
INSERT INTO main.experiments( extra_identifier, experiment, treatment ) VALUES ('CuratorBookCovers', 'EREADER_CURATOR_BOOK_COVERS_307723', 'T2');
and single reboot.
Code:
sqlite> select extra_identifier, poll_interval from extra_identifiers;
DecanterChromeV1|30
KINDLE_EINK_REMOTE_LICENSE_RELEASE_V2|0
CuratorBookCovers|0
sqlite> select extra_identifier, experiment, treatment from experiments;
DecanterChromeV1|DECANTER_CHROME_V1_FALLBACK_328845|C
DecanterChromeV1|DECANTER_CHROME_V1_MALBEC_FALLBACK_370896|C
CuratorBookCovers|EREADER_CURATOR_BOOK_COVERS_307723|T2
sqlite> .exit
I think for my case, the extra_identifiers table has no CuratorBookCovers value (PK for experiments table).