Interesting thought. Did some googling on how the system looked like originally, but I don't think many will have used the flexplate that's needed for this.
I have 3 A518 tranny's and 2 of them have a provision for the sensor, but none of the transmissions came with the proper flexplate. Which is kind of logical ofcourse as they stay mounted on their original engines normally.
I've eyeballed the hole in the bellhousing before and how it would come in handy for possible use as a cranktrigger, but until just now I had never seen the proper flexplate for this setup.
If I had known earlier I probably would have located one and mounted one on the 318 in my Dart.
I have another Overdrive-project in a '57 Chrysler with a 354 poly motor which could benefit from the cranktrigger idea.