Sure, I was imaging you parsing the dhcp_staticlist nvram variable and adding iptables rules for those clients if DNSFilter mode is set to Router and no other client-specific DNSFilter rules are defined. Just an idea, but I haven't even seen the new feature yet.
A bit messy since you'd have settings scattered over two separate pages, so it would require some proper planning before something similar got implemented. Like possibly just a toggle switch on the DNSFilter page to enforce the dhcp_clientlist parameter.
I haven't given any real thought yet to how to enhance this new feature since it's too recent.