The solution is to configure the option through the CLI. Log in using either telnet or SSH (whichever is enabled), and type:
srv dhcp option -e 1 -i 1 -c 66 -v http://192.168.1.10:5000/provisioning/ajehdukzidueh/cfg{mac}
Take particular care to the -i option, which defines the LAN subnet to be used: in this case, 1 references LAN 1 on the Draytek.
