> Does anyone know whether it is possible to use PowerDNS to build a persistent DNS cache?
What exactly do you unterstand to be a "persistent DNS cache"?
Ideas come to my mind including querylogging, automatic ressource
record extraction, revision control systems, lua and/or pipe backends;
so probably there will be (possibly very high-maintenance) possibilites
how you can make PowerDNS do what you are thinking about; but without
knowing what exactly you are trying to achieve, there won't be any useful
hints on how to get there.