@hyde
You can install dnscrypt-proxy and set pihole to use it as dns resolver, then in dnscrypt proxy you can specify what upstream resolver you want. Just set dnscrypt-proxy to listen on other port than 53. You have guide here:
https://github.com/pi-hole/pi-hole/wiki/DNSCrypt-2.0

There is also automatic update script for dnscrypt-proxy here:
https://github.com/DNSCrypt/dnscrypt-proxy/wiki/Updates