It's a very interesting thought that America might have revolted over the abolition of slavery
if they hadn't gained their independence before it happened. I'm sure that if the states were willing to revolt against Britain because they had a few taxes imposed upon them against their will, that they'd definitely revolt if Britain tried to force them to comply with something that would have such a massive impact on the American (considerably labour based) economy. And even if the northern states were still as (comparatively) progressive as they were at the time in our history, they'd probably side with the south in order to make a stand against British influence. Just think, instead of a nation divided by slavery, it would be a nation united by slavery.
I wonder how different America would be today if the union was founded on a basis of oppression, rather than principles of "liberty", and "freedom from oppression".