In order to maintain dnsmasq's DHCP lease information persistent storage, such as USB is needed. This isn't a scripting method/example, it may not be what you're looking for.

Navigate to DHCP/DNS under the Advanced menu. In the Dnsmasq Custom configuration field, enter:

dhcp-leasefile=/opt/var/dnsmasq/dnsmasq.leases

Use a path relative to your own system. Click Save.

Among other situations, when devices are connected to a switch they won't lose network connectivity if router is down (rebooted). They won't show as having an active lease in the device list until their current lease expires and request a new one. FreshTomato will just continue where if left off.