LuCi -> Network -> Hostnames is linked to kresd in Turris OS. So you can add an entry like this:
„turris“ - „Local ip of your turris“, e.g.:
turris - 192.168.1.1.
turris.lan will then be resolved to 192.168.1.1.
It may be needed to restart resolver („/etc/init.d/resolver restart“ or simply reboot Omnia)