My Piet will sometimes trigger just from cocking the bolt. I think it was just my mounting, I used some sub optimal velcro.Sorry, just saw this post.
I have not experimented with the external trigger enough to tell exactly how it works. I will say the JKL is sensitive enough that it will trigger from attaching the magnet to the trigger show though; that force would be 90 degrees to either the cross- or length-wise recoil impulse which you would get from "normal" mounting. No idea if the Piet is the same.
Edit to add: A post on Calguns forum stated that BrunoShooter was slated to get something like 20 LabRadar units in within the last couple of days. If you're still looking for one, might want to call them. The poster had ordered 2 units, and said they were on their way to shipping.