It's way easier:
example LAN segment is: 192.168.1.0/24
create static arp entry to resolve IP -> broadcast
/ip arp add address=192.168.1.254 comment="" disabled=no interface=LAN mac-address=FF:FF:FF:FF:FF:FF
create a NAT rule to forward packet on specific to this IP (effectively sending it to broadcast MAC)
/ip firewall nat add chain=dstnat action=dst-nat to-addresses=192.168.1.254 to-ports=7 protocol=udp dst-port=7000
tell your WoL client to send a packet to <your IP> port 7000. packet comes in, get's forwarded to broadcast MAC out LAN interface to port 7.