None of the Founders were champions of slavery as a cause - even those that personally owned slaves, such as Washington and Jefferson, characterised it as a temporary, necessary evil. The John Calhouns would come a generation later. The key is to have it happen at some mandated point in the future; slave trade abolished in 1808, and slavery itself abolished - say - 1828 (which all of the founders could merrily agree to because they expected to be dead). In return for this, the southern states need to be thrown an immediate bone - such as counting slaves as full people for the purposes of representation but 1/2 people for the sake of taxation.
There will still be a Gawdawful fight over it as the deadline looms, but your odds of success at that point become high - scream as he might, Calhoun won't be able to ram a Constitutional Amendment through.
I think you have the best way to go, at least at the Constitutional Convention. Slavery phased out over a period of years. I am still wondering if you would still need a POD as I suggested? Greater African-American contribution during the war, leading to more tolerance and sympathy for blacks in recognition of their contribution to independence.