Not so sure about that @cheebdragon .
On the one hand, Washington knew of the Northern colonies dislike for slavery. And he knew that Jefferson was trying to NOT allow slavery in the Declaration.
But- being a Virginian himself, and a slave owner besides (have you visited Mount Vernon), he would have been economically in favor of retaining “states rights” which is ostensibly what the Civil War was fought about.
But this is all silly speculation. Washington died in 1799. The civil war was in 1861. The Missouri Compromise (slaves states versus free states) was signed in 1820. In between 1799 and 1861, there was the War of 1812, the Toledo War (1833), the Louisiana Purchase (1803), and dozens of other events that shaped the US.
So you can’t just say “the north” or “the south” without taking into account the various historical factors that might have changed Washington’s mind either way.