OK, I see now that my first attempt (above) at the command line was overly literal in following the directions and consequently invoked an earlier version (not v1.2) of the .exe file.
So, I fixed that, and also I shortened the file path in case that was a factor. This time I invoked it with:
C:\mwpv1_2>mwp1_2 13,115200,123,c:\mwpv1_2\wpn1.hex
and it worked!
There's no feedback on the command line itself as to whether anything happened. However, it does launch the GUI app (and quickly shrinks it) with the proper parameters, runs it (diagnostic mode is off), and it closes upon exit.
So, it's a first step. My main interest isn't in that, per se, but in the virtual com port that might (?) be added in the future. i.e. using an <ip-address>:<port> instead of a serial com port on whichever computer is sending the hex file to the Wireless Programming Gatway. Actually, if that were in the GUI, it would be even better, but I'd use it wherever it is. Then there would be an excellent chance I could marry an esp8266 (acting as the virtual serial port) with a Moteino Wireless Programming Gateway and thereby have a cordless (and portable) Wireless Programming Gateway. No dongles anywhere!