The example on that page is expecting "postition" to go from 0 to 1 across the view port going from left to right, and top to bottom.
What you can do is pass a varrying vec2 from the vertex to the fragment shader.
gl_Position = gm_Matrices[MATRIX_WORLD_VIEW_PROJECTION] * object_space_pos;
pos = gl_Position.xy * vec2(0.5,-0.5) + 0.5;
With an orthographic projection, the extent of the view port will be contained within the values of gl_Position.xy of -1 to 1.
If you want the effect to cover the whole view port, you need to draw something across the whole view port.