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

Navigate to DHCP/DNS in the Advanced menu. In the Dnsmasq Custom configuration text area enter the text

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 loose network connectivity when the router is down (rebooted). They won't show as having an active lease in the device list until their current one expires and request a new one, now FreshTomato will just continue where if left off.