I won't have time to finish it up until next week at the earliest, but I'm in the process of making a shutter trigger which uses a phototransistor not mounted externally, but mounting it so the phototransistor is IN the viewfinder- the idea being that I can set a particular scene and the camera will trigger with any large (adjustable) change in the overall brightness "seen" by the phototransistor. My intended target with this is to get some wildlife shots, but it would work for lightning also-- anything that will change the overall illumination through the lens ought to do the job..