The problem with pan-Americanism is that it would almost inevitably end up completely dominated by the United States. I think it's very plausible to have pan-Americanism, but it would basically be the United States and some pliant Latin American elites ruling over everyone else. It isn't really in the interest of Latin America to become a colony of the United States in all but name. Although before the United States can do this in Latin America, they need to reduce and marginalise British interests. This is no easy task in the 19th century, the height of the British Empire, where the Monroe Doctrine's strength came largely from the fact that the British also liked the idea for obvious reasons and thus enforced the doctrine for the United States. So I suppose you need an Ameriwank where the United States is near-irrevocably poisoned against the British, fights a third war against them in the 1840s (either winning or getting a stalemate out of it), resolves the issue over slavery without a Civil War, and has a huge military buildup starting in the 1850s onwards.