No matter what tool you will use, that tool needs access to the devices behind the one with public IP address. From your explanation, I assume that Rancid (or the new tool) is located somewhere remote and the connection to the devices that has to be managed is done over Internet.
In this conditions, I would suggest that at least temporary to configure static NAT inside on the device with public IP address and to forward some ports to the devices behind it. E.g. for ssh connection
port 22 - remain for device with public IP
port 2221 forward through NAT - 1st device with private IP behind the device with public one
port 2222 forward through NAT - 2nd device with private IP behind the device with public one
and so on...
If you have problems with NAT configuration let us know.
Even is you have a Cisco tool, like CiscoWorks, and configure the devices with private IP address with SNMP to reach the CiscoWorks system, this will not know how to get back to those devices. So, again back to NAT.
If this helps please rate!