Confirmed, this is the secret sauce! And to be clear which ID is used as "the specific VLAN ID" doesn't appear to matter, which leads to something of a decent workaround:
I set my VLAN1 back to "tagged" on both CPU and PLC switch ports but added a new VLAN1234 which, as described above, needs to be set as "tagged" for the CPU and "untagged" for the PLC (/LAN 4) switch port.
Since I had to explicitly add an interface anyway (i.e. eth0.1234 didn't magically become available just because I configured it on the switch) I named it "plc" and then set it to bridge over the switch eth0.1234 device.
Et voilà!
Now [after I switch which package is installed…] I am able to run plcstat -i br-plc -t and get back info about all PLC transceivers!
Now if only this router had enough overlayfs space that I could install more than one of the open-plc-utils-* packages at once 