If a secondary or tertiary DNS could be set with PH DHCP you could have a simple PAIR (or triplet) of PHs running with each having part of the DHCP scope to hand out and each pointing to the same upstream DNS server, same gateway and same subnet mask. That would create a simply sort of PH ‘fallover’ cluster.
With several PHs on the same LAN with each handing out addresses as part of the same DHCP scope like 5-50, then 51-100, any other PH could take over if the other one should go offline.
If only a single PH is on the LAN then the second DNS should automatically default to 0.0.0.0.
This can be done if your router can handle DHCP with internal DNS settings, but a lot of routers can’t.
I know it runs the risk of a user setting an external IP as the second DNS and rendering PH useless for anything using that second DNS as resolution. But I think it would be worth it for hundreds of users out there that want a second PH for redundancy.