On another topic, someone says than a terminal command can force dhcp to send wanted options on dhcp requests…
http://forum.mikrotik.com/t/feature-request-force-sending-of-dhcp-options-to-clients/84291/1
Force DHCP Server on RouterOS 7.1+ to send all DHCP Options specified on DHCP Network
ip/dhcp-server/option/set force=yes