Why don't you just do as suggested? Set NAS to DHCP and check at the ER which MAC the NAS uses and reserve the IP for that MAC. DHCP works MAC-based - by design. You could use a random MAC or maybe the FF-FF-FF-FF-FF-FF broadcast MAC but I don't really see why and I'm not sure if that's safe to do. Again, this isn't necessary.. It is up to the network administrator to document the network settings properly, like the static IPs of servers, printers, etc... You could add a address group with the NAS IP and name it "NAS". In the firewall and/or NAT tabs you can then use the address group instead of a hard coded IP address.
You have 2 options:
1) use DHCP @ NAS and reserve IPbyMAC @ ER
2) use static IP outside DHCP range and don't bother about DHCP reservation....