dd-wrt and Tomato use dnsmasq to allow easily adding manual DNS entries for reserved DHCP addresses. This is useful for adding names for devices with static IP addresses that don't use DHCP (e.g. a cable modem or the router itself) and for allowing a device to be addressable by multiple names (e.g.
Synology's SRM also uses dnsmasq but doesn't provide equivalent UI. Hostnames can be set via Network Center > Local Network > DHCP Reservation, but hostnames assigned there won't take effect until the device requests a DHCP address, so it is ineffective for devices that don't use DHCP. It also allows assigning only one hostname.
Is there a way that I can add manual DNS entries?