Okay, let's talk about the egg solution first. It's interesting that nobody posted they had used that method, they've just *heard* about it.
Can you really carry a raw egg on a difficult horse, riding outside the ring and at the exact right moment, fish it out of your pocket and crack the horse between the ears with it? Assuming the horse's neck and your arms are the correct length? Really? How many times (and how many eggs?) are you going to have to go through this exercise before you get it right? What if the egg gets broken (highly likely) before the horse rears?
As far as cracking them between the ears with a stick, this *can* work if handled exactly right, by which I mean you must time the stroke as the horse comes up of the ground and near the top of its rear for it to be effective. If it happens too early or too late it doesn't work, and if you hit the horse someplace other than between the ears and without the correct timing, yes, it qualifies as abuse. So anybody who's confident enough that they can execute this manouever correctly while staying on the rearing horse, have at it.
Finally and more on point, as kevinshorses has said, why not just correct the cause of the behavior?