First, go to the directory for NullDC itself and open up nulldc.cfg in Notepad. Change the following options:
Now go into the Dreamcast options for ECC, set the emulator to NullDC if it isn't already, and give it the following command line:LoadDefaultImage=1
The only option checked should be "Escape Path."-config ImageReader:DefaultImage=%ROM%
Presto - command line launching for NullDC and Dreamcast games. I found this method somewhere on Google a few days ago. I'm honestly not sure why I never happened upon it before, but it's way better than using the AutoIt script.