But my understanding is that slavery had been abolished in other places (like Britain, and thereby extension Canada) by that time. So there still would have been the idea of the alternative out there, and even in the days of perpetually sketchy snail mail, the US did not exist in a vacuum.
And I mean, other human rights issues became more and more prominent within the next fifty years or so of the Civil War, like labour and women's suffrage. I mean, I know there are unfortunately lots of people in history who want rights for one group but would happily see others kicked to the curb (one of the heroes of women's suffrage in Canada, for instance, was apparently a fan of eugenics), but I find it hard to believe those various movements would have no influence on each other.