Nice that you figured it out yourself!
You need to indeed just send the raw binary data.
A couple of simple dos commands would have also sufficed:
mode COMXX: 9600,n,8,1
copy /b [FILE] COMXX:
Where you would replace [FILE] with your file and COMXX with your com port.
Or you could create a batch file, on which you can drag the ROM image (not tested, but you get the idea):
@echo off
mode COMXX: 9600,n,8,1
copy /b %1 COMXX: