[troyb]

Unfortunately you can't really create a search shortcut via the command line. The shortcuts are stored in your preference file (which you can use the defaults command to edit) but they're a dictionary inside a dictionary which as it probably sounds, is a bit complex. The defaults tool just isn't capable of working with these situations.

That said, you can quickly add search shortcuts to OmniWeb without using the command line. Just control click inside any search field and select "Add search shortcut" from the context menu.