Our provisioning happens via an interaction of the dhcpd-plugin, and the isc dhcp3 server. It's basically split with the rules outlined in the Advanced > Network > Network Settings page, with the following implicitly:
If a device is added to the device tree, that has an IP and MAC address (see the Advanced > configuration > network devices), then that MAC address is given that specific IP address via a static mapping. You should not modify the DHCP configuration file directly, as your changes may be overwritten when the router needs to be reloaded.
So if you need a device that absolutely needs a static IP mapping, then add a device to your tree. If there is no template, just make one, even if it is a "dummy" template with no device data, or pnp information.