Ignoring all the ASBs and the storyline of Guns of the South, I believe Turtledove did make one good point about Lee when it came time for him to decide to support abolitionism, which could translate to a standard AH timeline: If the Confederacy did win its independence anytime after the Union began allowing African-Americans to fight as soldiers.
Now, we all know that such would be simply next to impossible given the sheer strategic and economic advantages the Union possessed when it came to dealing with the Confederacy, but if someone could construct a timeline with the right sort of POD that might allow such a victory to occur post-Proclamation, then Lee -- who despite being a slaveholder and a very conservative man, was also a practical realist -- might consider putting his support towards an abolitionist movement in an independent South.
Note: I said "if!"