Now that I am no longer aiming for a wrist strap sensor location, I have built a simple prototype strap to use with the pi smartwatch in order to wear it on the wrist. This prototype could and may be used to experiment with a strap control method, time permitting. 

This is simply 3 strips of thin pliable leather sewn together along the edges. 

I did originally try 2, with a piece of rubber in-between, like so:

However, this turned out to be too stiff for good manipulation around the wrist. And too stiff for the hook and loop fastening system I have gone for with this strap design - it would pull itself loose.