READ THE LAST PARAGRAPH FIRST IF YOU DON'T WANT ALL THE DETAILS.
Flint does not supply localnet DHCP or firewall. Localnet DHCP is handled by a completely different (not Flint and not DD-WRT) DHCP server. WAN firewall is handled by DD-WRT. Although, there is a firewall rule automatically installed by GL-Inet in the Flint that isolates the guest network. (The rule says source zone guest (no IP), destination zone LAN, IP my locallan address, dROP.)
Flint is (without me apparently configuring anything) supplying DHCP for the guest network only.
Since this is all working, and guest devices have internet access, I presume the Flint is somehow automatically handling guest->WAN forwarding and is also handling any NAT needed from the guest network. Flint is not handling NAT for the localnet, that's in DD-WRT.
The difference in the traffic rule you are proposing from what we tried yesterday is that the destination zone is WAN instead of LAN. I gave it a try. The result is the same as yesterday - I can ping the printer from the guest network, but not print.
Just as a test, I tried disabling the rule that blocks guest access to the localnet, and right away I was able to print. I then re-enabled that rule, and tried the forwarding rule just for the printer both above and below the blocking rule but was unable to print either way. I then tried adding !printerIP to the destination address for the rule that blocks guest from localnet, but still no printing.
So, it's clear that the rule that isolates the guest network from the localnet is what's blocking printing. But I'm unable to figure out why I can't override that rule for the printer IP alone.
So, as a last gasp, I tried disabling all the guest firewall rules in the Flint, editing them without making changes, saving all the rules. And MYSTERIOUSLY it started working fine! This was with switching the destination zone back to LAN from WAN.
Thank you for your help!