This isn't before the Revolution, but if Jefferson's 1784 proviso banning (after 1800) slavery in all the western territories (not just those north of the Ohio) had passed, it might not have kept slavery out of the enire Southwest, but it might have had a real effect, especially in Kentucky and Tennessee:
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"While rejecting the notion that Jefferson's free-soil clause would
have had no effect, I do not mean to suggest that the clause would
necessarily have prevented slavery from becoming firmly established
anywhere in the Southwest. It would most likely have affected the
northernmost and most mountainous portions of the Old Southwest,
in which slavery never became as widely and firmly established as it
did in Alabama and Mississippi. Kentucky was not part of the original
Virginia cession, and the Bluegrass State never went through a territorial
stage, passing instead directly out of Virginia into full membership
in the Union in 1792.184 North Carolinas cession of Tennessee
did not take place until North Carolina ratified the Constitution in
1789, at which point Congress took over administering the Territory
South of the Ohio, leading eventually to Tennessee's admission in
1796.185 Regarding the remainder of the Old Southwest, it is all but
certain that South Carolina and Georgia never would have made
their cessions had Jeffersons provision remained in force, and even
without an anti-slavery provision in place, conflicting Spanish claims,
Indian wars, and complex Georgia politics involving various factions
of well connected speculators with conflicting claims to Indian lands
in the Yazoo delayed establishment of the Mississippi Territory
(comprising the future states of Mississippi and Alabama) until 1798.186
At that time, Congress considered but rejected legislation that would
have prohibited slavery in the new territory.187
"For the future states of Kentucky and Tennessee, however, approval
of Jeffersons anti-slavery clause in 1784 would have created a
period of substantial uncertainty, and any uncertainty worked against
the immigration of slaveholders. Demographic history prior to the
first federal census of 1790 is inexact, but even in 1790 Tennessee
(The United States Territory South of the Ohio) had a black population
of only 10.6% (of whom 90.4% were enslaved), similar to New
Yorks (7.6%, of whom 82.1% were enslaved) or New Jerseys (7.7 %,
of whom 80.5% were enslaved), where slave owners lacked sufficient
clout to prevent emancipation by political means.188 Whether slave
owners would have streamed in to Tennessee in the 1790s with
emancipation scheduled for 1800 is open to doubt. While Kentucky's
black population for the 1790 census (conducted in the district which
was then still part of Virginia) was already 17% (99.1% of whom were
enslaved),189 it is questionable whether slaveowners would have risked
establishing themselves in the region after 1784 with Jefferson's
clause and its 1800 deadline looming over all territories to be
ceded.190 This marginal uncertainty may have been determinative,
and even in 1792, state constitutional sanction of slavery was only
achieved after a hard fight in the convention.191 Immigrants into
Kentucky between 1784 and 1792 could not have foreseen that Kentucky
would never pass into territorial status, or that they would win
constitutionalization of slavery at the time of statehood, even if the
eventual separation of the region from Virginia was expected by the
time of the Territorial Governance Act. All this is of course
hypothetical, but while I cannot show that Jefferson's provision would
have reduced the eventual number of slave states, neither can Finkelman
show that it would not have..."
William G. Merkel, "Jefferson's Failed Anti-Slavery Proviso of 1784 and
the Nascence of Free Soil Constitutionalism"
http://scholarship.shu.edu/cgi/viewcontent.cgi?article=1115&context=shlr