That is similar to what I had in mind, but using a microswitch that has the roller on the arm.
I will take a look on the Pi solution for the run out sensor. :)
The 3rd pillar is a good idea. Maybe in the middle of the Z travel on one of the back sides. The other idea I had was to move the Z stepper to the top and add another screw to the other side with a belt system. That way it does better with having weight on it.