If you want to build anything for an existing image, you need to get back to the pre-cupcake merge. And irc user haakjes figured out how :)
Its simple and elegant. And relies on jbq using unimaginative merge changelogs :)
repo forall -c 'COMMIT=`git log | \ grep -A 10 "Merge commit .remotes/korg/cupcake" | grep '^commit' | \ sed -e "s/^commit //"`; \ git reset --hard $COMMIT'