You seem to be suggesting that ISPs run stealth slaves for these kinds of zones.
Not really. In today's world such simplistic solutions don't work.
For zones that are being made available on anycast servers, ISPs may be able to lobby/pay the zone operator to install an anycast instance in their network. However, in general, the days of ISPs being able to set these things up on their own and see benefit from them are past, in my opinion.
I believe that there are still some things that ISPs can do which cannot simply be bought on the market. For instance, most ISPs runs simple caching servers for their DNS queries where they keep any responses for a short time before deleting them. It's so simple that it is built into DNS relays as an option. An ISP could run a modified DNS relay that replicates all responses to a special cache server which does not time out the responses and which is only used to answer queries when specified domains are unreachable on the Internet. For instance, if you specified that all .es responses were to be replicated to the cache and that your DNS relay should divert queries to the cache when .es nameservers are *ALL* unreachable, then the impact of this type of outage is greatly reduced. You could specify important TLDs to be cached this way as well as important domains like google.com and yahoo.com. The actual data cached would only be data that *YOUR* customers are querying anyway. In fact, you could specify that any domain which receives greater than x number of queries per day should be cached in this way. The volume of data cached would be so small in todays terms that it only needs a low-end 1U (or single blade) server to handle this. Since nothing like this exists on the market, the only way for ISPs to do this is to roll their own. Of course, it is likely that eventually someone will productize this and then you simply buy the box and plug it in. But for now, this is the type of thing that an ISP has to set up on their own. --Michael Dillon