So, remove the info from /etc/config/etherwake and create a DHCP reservation which will then sit unused?
At the moment, I have a "domain" entry: I've tried adding a "mac" option but it is ignored, as I expected.
The problem with a reservation is that the hostname will remain unknown forever, because the NAS will never claim it. That's why I added a "domain" entry for static hosts.
You can leave the config/etherwake as it is. Create the DHCP static lease, which will be unused, but it doesn't bother anyone. You should leave the domain entry as it is.
I had hoped that I could avoid triplicating the same information
Especially because the dhcp pool starts above the reserved range for static hosts (below 10) so it's a probably a good idea to use a dummy name. Fun fact: the dropdown menu still maps the MAC to the correct hostname (domain entry)... I suppose that's because the lease is unused.
You can use the same name in the dhcp host entry with the domain entry. Also it doesn't matter that you have dhcp pool range higher, you can reserve static leases outside of the main pool.