In any case you would have had the Federal Government telling the South what to do with their slaves. In other words the North telling the South what to do. Absent a Constitutional amendment, impossible to get ratified, it would have had to have been done through legislation. So how long do you think it would have taken for the South to sit back and accept such a violation of state's rights without rebelling?
This would have solved other problems that Reconstruction caused. My Grandfather who was born in 1888, and kindled my interest in the WBTS claimed that Reconstruction was far more devastating to the south than the war was.
Like what? Accepting the freed blacks as citizens with the same rights as whites? Allowing blacks to vote? Heck, you didn't see that for free blacks down South before the rebellion, what makes you think it would have happened regardless of the means of emancipation?