To those who may read this in the future, this is the rule to make a LAN device DMZ. Make this the last rule in the Port Forwards, at the bottom of /etc/config/network -
config redirect
option target 'DNAT'
option src 'wan'
option dest 'lan'
option proto 'tcp udp'
option src_dport '1-65535'
option dest_ip '192.168.xxx.xxx'
option dest_port '1-65535'
option name 'DMZ'