Honest question: Has anybody used telemetry from a personal wearable device in the context of using it as proof of testimony, or something akin to that?
Right. There was no collision, the cyclist fell because he had to break hard to avoid it, but the car driver did not even notice what happened. The license plate number was visible in the video.
On a related note, could this backfire if, e.g., you're compelled to give someone your video so they can make a case against you? In a criminal case an American could probably exercise the 5th amendment, but what about a civil case, or insurance claim?
The 5th Amendment wouldn't apply. The 5th Amendment protects against self-incrimination, not incriminating evidence (like a video recording from you dash cam) except in very specific circumstances that aren't applicable here.
5th amendment for the most part covers only your testimony and pretty much what's in your head.
Any physical evidence can be accessed through a search warrant, and in some cases if it's on your person (e.g. cell phones) without it.
Not sure what's the case of a car, but as far as i know if the car is impounded everything in it is admissible evidence.
If the car wasn't impounded then the court can order you to produce the recorded footage as evidence, if you refuse then you can be charged with contempt of court, and technically put in jail indefinitely until you comply.