Licensing is supposedly done by calling prophcfg from the program directory with parameters. prophet.chm located in the same directory provides some information on this.
However when using -Q there still is a UI and the path to the licence file -L will also be ignored. Using a path in quotes will even result in a runtime error.
You can get around all this by just copying your license file (prophet.pli) to %AllUsersProfile%\Applications\SunGard\Prophet73