Maybe you can set the camera on B.Then put a piece of cardboard over the lens and fire the shutter off. Remove the cardboard after the shutter is open and put it back over the lens after the correct exposure time. Then release the shutter. I believe that this would take shutter curtain problems out of the picture. You could try to shoot a roll with some exposures using the cardboard method and some with normal B and see if they are different.