There are two sides to the problem including one obvious omission, bicycles. Also, some of the extreme hypermilers had suggested turning engines off of even non-hybrid vehicles. Now if it comes down to another maintenance item that gives the hybrid owner no value but an additional maintenance expense and requires government inspection for maintenance, it won't work. The parts will fail and not be repaired leaving the blind with a false sense of security and worse, the driver assuming "they'll hear it" complacency.
It is not a trivial problem but just "belling the cat" would be the wrong approach:
http://www.bartleby.com/17/1/67.html
In my best, oldest mouse voice, "We'll open a position for a blind person to affix the noise makers to all hybrids, bicycles and any cars not fixed so the engine can not be turned off in operation."
But if this effort leads to an audio safety system for collision avoidance, it would be a good thing for everyone. What is needed is a 'win-win' system that adds value to everyone. Right now, it sounds like "bell the cat."
Bob Wilson