Trice, I know firsthand of exactly one case where a rider entered Mexico with absolutely zero documentation. No visa, no TVIP, nada, zip, zero.
I explained, patiently, the insanity of this.
He mentioned he had ridden Ethiopia and a few other interesting places and said that he would worry about things if and when they happened.
He actually made it into Guatemala to the surprise of everyone.
It can be done, but man o man the way the planets have to line up is beyond the luck of most, I believe.