Activate DHCP when Interface goes down

how can I activate the build-in DHCP Server in my OpenWRT-box when a certain interface goes down?
In normal operation the DHCP server is deactivated, cause this service is handled by a virtual (ESXi) pfSense instance.
In case of any problems with that, I'd like to have a simple way to have a workaround.
Pull out the pfSense jack, for example, would work fine.

My OpenWRT router is directly connected to the ISP.
The necessary networks (LANs, transfer) are connected to the one-legged ESXi box via a VLAN-trunk on LAN1.