I want to apologize for this crazy feature request.
I just ask to our CTO what we use over serial port? And his anwser was "of course telnet". And them I realized, my request was a nonsense.

Sorry

What file transfer protocol are you using over serial port?

Like the last Putty version it will be very cool to suport serial port interface (COMx) in WinSCP.
We work on the embedded hardware wich they have only serial port available before running Network with Ethernet. Some time we must update the firmware by the serial port.