The problem with this is that there may not be a single zone file. Either because of location aware resolvers, or because the entry won't exist until it's requ

The problem with this is that there may not be a single zone file. Either because of location aware resolvers, or because the entry won't exist until it's requested. This means that caching the whole zone wouldn't allow for dynamic resolving and would require lots more caches at whatever granularity someone else wants for their local server architecture.