Pi-hole's embedded DNS server pihole-FTL is a tailored fork of dnsmasq. Unless upstream dnsmasq would add support for DoH, it is unlikely that this would be added to Pi-hole.
That's wrong.
EDNS0 Client Subnet (ECS) can convey information about a DNS request's originating network, up to the full IP address.
If your DoH proxy software would support sending ECS information, you'd only have to enable ECS parsing in Pi-hole, by setting EDNS0_ECS=true in your pihole-FTL.conf.
It's not entirely clear to me what doh-proxy you'd be using, but a precursory search suggests that there are DoH proxies that would support EDNS0, e.g. GitHub - tinkernels/doh-proxy: A DNS-protocol proxy for DNS-over-HTTPS providers, such as Google and Cloudflare
-edns-subnet string
Specify a subnet to be sent in the edns0-client-subnet option;
take your own risk of privacy to use this option;
no: will not use edns_subnet;
auto: will use your current external IP address;
net/mask: will use specified subnet, e.g. 66.66.66.66/24.
(default "auto")