I was thinking about ways that the U.S. might plausibly be 'better' in this regard, and I simply can't see it. As you say, their prejudices may lessen (slightly) but they're still a country in the 19th century that will nakedly try to impose their will on others for no other reason than "the strong do what they can - and the weak suffer what they must." I was initially wondering if maybe we could see an earlier attempt by the U.S., somehow, to try supporting native powers against encroaching Europeans but there are a lot of factors going against it (money will be needed at home for internal colonialism against the natives/Reconstruction/famine relief soon, the navy is still not up to snuff enough to ferry Americans that far, not to mention that even if this more radical civil war leads to a more politically progressive United States in the long term in the short term they're going to prefer Europe, even if they otherwise despise them, to other places). That said, a failed colonial venture (or a highly controversial annexation) is certainly an interesting idea; at the very least I could see that effecting debates on what to do regarding the tribes that haven't quite been crushed under American boots yet.
Or, I dunno, even if anti-colonialism isn't readily accepted by Washington as a government policy then perhaps men radicalized by the Civil War do lend their military services abroad to places that want to get rid of their own reactionaries. Some sort of literary movement ITTL by Americans that's primarily military adventurism set in exotic locales, perhaps? I think this generally would gel with the ethos of the story.
