That's actually what I was getting at earlier -- not would the Deep South support a policy of gradual emancipation in 1789 as such, but would they be thinking to preserve their property over a specific timeframe (say the next 50 years) instead of insisting that slavery in their states be untouchable in perpetuity? Actually, even more specifically, I was thinking that if they suggest a provision that the Constitution that not only can the federal government not touch chattel slavery in those states that have, but no such amendment can be passed touching said provision - if such an extreme counterbalance was sought, the other framers could very well object on legal grounds, saying that not allowing the Constitution to be amended in the future is a dangerous proposition, whatever the subject matter at hand. From there, it could be a simple matter of someone suggesting "How about we just say no such amendment for the next 50 years?", and everyone agrees that will work; then skip ahead to 1839, and low and behold, the idea of abolishing the institution once and for all has become pretty popular.
If the South thought this was even remotely possible they just wouldn't have ratified the Constitution in the first place.
The clause about the slave trade would serve as a warning. Everyone knew that "not until 1808" would in practice mean "as soon as 1808 arrives". But at least that didn't affect a State's internal affairs. This decidedly would have.
And then there's the question of what would happen to the slaves themselves. If they are no longer slaves, but making them citizens is unthinkable (even to most of the North) then what the heck are they? At best, expect a "slaves in all but name" situation.