you can easily create such a sun ray shader with the help of a radial blur shader. All you need is a rendertarget which stores all objects as plain white color and the sky as black color. You then pass this to the radial blur shader, do some calculations and you are set.
Have a look at the radial blur shader in shade-c. It actually does exactly what i was talking about above. I just commented it out, as it wasn't very user friendly at the time i released the last version of shade-c