Yes, that would be very interesting. As a hiring manager, I wouldn't make a decision based solely on that information, but I definitely would use it if it was available.
From the feedback that i have got from this thread, however, feedback is a very personal thing and users who really want to improve themselves don't have a solution right now where they can privately request their coworkers(former or present) for this information and not get potentially penalized for it (if negative) by their existing managers in the performance review meetings.