It depends on the car's ECU, and your technical skill-set. For a car like a Mitsubishi Evolution, there is a largish community re-flashing: http://forums.evolutionm.net/ecuflash-179/ is a nice place to learn and understand the craft.
I think if crash investigators lack the ability to dump the ECU rom and have it compared to an original, they are somewhat incompetent.
Sure, but only if they think of it. I doubt "hacked ECU" is at the top of the list when they start investigating causes.
On the other hand, I guess once they figure out you crashed without braking (no skid marks) but were still conscious when you did so (you've probably tried swerving to avoid an accident) they'll pretty quickly narrow it down to complete brake failure, which I guess has a much smaller list of possible causes, especially if the brakes look physically okay.
I think if crash investigators lack the ability to dump the ECU rom and have it compared to an original, they are somewhat incompetent.