A radical solution (as suggested by DuckieTigger some time ago via PM [no, I didn't forget it... thanks for the idea!]) is to delete the /etc/uks/pubprodkey*.pem files. At least in theory, this should prevent the Kindle from installing updates signed with Amazon's official key - much like it prevents the installation of custom packages when the pubdevkey01.pem is absent.
I'm not 100% sure though, so I'll give this a shot when I get home and let you know.
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