Use ATtiny85 to perform initial config on a Cisco router using its USB console port
It's been a couple of decades since I was programming Cisco routers, firewalls and switches but I appreciate you pain. Like @Gerben says, you need a bigger processor, an ATtiny85 isn't going to be able to hold a lot of config and even the basic configs I used were about 4 to 6KB.

If there is anything in the configs that is different, you idea is going to hit problems.  Are the device names and passwords all the same? In which case you probably want the engineer to be able to set some configuration data in you "programmer", could they set this specialisation data using their mobile phone?

I doubt you will have any problem connecting an "Arduino" via serial (RS232) to a Cisco router. However you now have to encapsulate the serial in "USB", which I don't think will be straight forward for an "Arduino".

I can't think of an option that doesn't require power, but as an alternative to your idea what about using a Raspberry Pi to connect to the router via USB. Stick a touch screen on it and a battery pack to power it.