I was about to propose to add it to the Settings / DNS / Upstream DNS Servers list of the web interface, but I read that, unfortunately, that list will not grow anymore.
OpenDNS Family Shield is reachable also via IPv6. This fact is not googled immediately, so it would be useful to have its IPv6 addresses mentioned in the documentation. I can prepare a pull request, if needed.
Note that configuring an IPv4 address for a OpenDNS Family Shield server for Pi-hole would be fully sufficient, as long as Pi-hole's host would be able to access that IPv4, and plain OpenDNS also claims to do filtering, if not for adult content.
Pi-hole's documentation already mentions the Family shield IPv4 addresses, so users already could use those now to make use of OpenDNS Family shield servers.
Wikipedia's English OpenDNS article has a nice table on OpenDNS's DNS servers for both IPv4 and IPv6. I wonder why OpenDNS themselves do not provide an equivalent comprehensive overview...
Pull requests are always welcome.
In this case, I think a pull request would increase the chances of IPv6 Family Shield addresses appearing in the documentation.