Happy to help Dave. It brings back fond memories of Gene. To answer your questions:
Bridge Length: It varied, depending on the shot --- shorter for short shots and longer for long shots. I would say his bridge length was the same as it was before he started throwing the cue.
Back hand grip: He cradled the cue with the last three fingers, sometimes the last two on delicate shots. He took all of the pressure off of his thumb and index finger and kept them pointed down. They barely touched the cue. At first he released the whole cue but as he experimented he started perfecting the throw while hanging on to the cue only with his pinkie to avoid having the cue foul other balls. However, on some shots he continued the total release. It depended on the shot. I remember that he would compeletely let go on tough cut shots and I would watch him make table length extremely low percentage shots over and over with a total release. He hung on to the cue with the smaller shots and those where he had a greater margin for error.
Rail shots: He did throw on those shots and allowed the tip to rise. That didn't bother him. He kept his bridge hand secure, as he did with other shots as well, in order to control the falling cue stick. On rail shots, the tip would rise and often the cue would slide to the floor along side the table in an upright standing position. It was his bridge hand that controlled that and kept the cue from hitting the floor flat. He was constantly figuring out new techinques to allow him to throw the cue and still make legal shots.
I remember in a WPBA tournament, I was faced with an extremely thin cut shot near the head string along the side rail, practically 90 degrees, and I knew that the only chance I had to make the cut and draw the cb all the way down table for the next shot would be to throw the cue. So, I threw it. It hit table and then fell on the floor with a loud noise and the crowd all thought I had some kind of accident with the cue, but the shot went and the cb drew all the way down the diagonal of the table for perfect shape. I just pretended it slipped out of my hand by accident. Haha.