I'll tell you what we do in hardware to get test coverage that is worth way more than 100%: we do fault injection. I have never heard of anyone trying to do the same in software, but it would be interesting.

Here's how it works:

for each statement in your code, break it, then run your entire test suite. If none of your tests have uncovered a problem, mark the code as uncovered. Repeat for each code statement.

It's crazily expensive to compute, but it's so much more accurate than to say that a line of code was hit or not. It measures whether the code had a functional impact on the overall behavior.