How would devices use the obscure DoH IPs, there would have to be a method to update/lookup said IPs. That same method could be used to keep an up to date block

How would devices use the obscure DoH IPs, there would have to be a method to update/lookup said IPs. That same method could be used to keep an up to date block list. Alternatively, the traffic could be subject to heuristics to identify DoH connections.