You could simply compare the angle of the sprite to the revers angles of the camera...

If the sprite <> camera.pan * -1 alter the alpha accordingly...

That would seem the simplest method to me. Of course you could use a little more code and get more info that might help you, take a look at this thread thread
and then reference the manual for vec_to_angle and associated functions.

Hope that helps
And no...not a shader i think