Rename the four video files that are in your Movies folder. Then when you launch demigod, it'll instantly post the game menu.
I used to do that but every time Demigod updates it re-downloads the movies which makes updating take longer and requires me to fix it each time I update the game.
You can create 2 bat files, one to rename from original to another name, and another one to rename it back before you start updating. I know it is a work-around and the option to set the commandline parameters in some kind of options screen would be nice.