Conditional forwarding to Bind on localhost at an alternate port?

[Bucking_Horn]

In that case, CF won't help you, as my first sentence applies:

@Bucking_Horn

Since you are already using bind as Pi-hole's upstream resover, there is neither need nor use to enable Pi-hole's Conditional Forwarding - provided that bind would actually know about local names (either directly or by appropriately forwarding respective requests itself).

AFAIAAO, ISC is strictly DHCP, i.e. it doesn't handle DNS, e.g. for some of Ubiquiti's EdgeRouter models, it is recommended to disable ISC and use dnsmasq instead for local DNS names. And I remember some reports for certain MicroTik models, where installation of a custom script on the MicroTik router was required to handle DNS record creation from DHCP leases.