One thought might be to use a laser/photocell to reflect off the film base at the sprocket location. Triggering in LOSS of refection would give the location of the sprocket hole.
Another thought would be to emulate the way the motion correction software works and use a photocell looking through the image to find the frame line. Combining this with the scheme to advance a given distance, the lack of a frame line would not be a problem, but the first time you find one it could correct the position of your pull-down drive. (Perhaps TWO photocells one frame line apart would be more resistant to geometric patterns in a given image?)
{just random thoughts on the problem}