I've dumped the cache, it goes to the pihole log, which is already on ramdisk.
This dump contains all entries, including the 0.0.0.0 replies. The idea is, to have pihole-FTL write the valid content (NOT 0.0.0.0 or ::) to an additional database (only if instructed by the command sudo kill -10). The entries in this database could than be used to generate an unbound prefill configuration file (not part of pihole, customization for advanced users), ensuring the data is available for use in the unbound cache at all time, thus speeding up the response time.
As I indicated, I'm currently trying to achieve this, using the unbound + Redis configuration, not yet sure this is benificial...