Horrible idea. Most companies have their policies of not giving references either way. Lawsuits over good references (for people who turned out not to be good) have occurred, and even though the probability of a plaintiff victory is low, who wants to deal with that pain in the ass? Most people are fine saying that someone was essentially decent and worked hard, but aren't going to go out on a limb to say that someone who no longer works for them is "EXCEPTIONAL".
Also, what if the person doesn't remember his boss's email or gets it wrong? What if his boss is three companies away by now? Lots of false negatives here.
I think reference checks are not essential for junior hires-- not having references is a warning sign, but I'd take a chance and decide fast on someone if necessary if he wasn't going to be in a managerial role-- but I'd require references from past subordinates (2 per company) from anyone being hired into a managerial role. I actually think subordinate and peer references are far more useful than managerial ones: if you want to see someone's true character, ask people he had power over.