One biographer of the Marquis de Lafayette wrote, "On no subject perhaps did Lafayette reveal the consistency of his adherence to the principle of liberty more staunchly than on that of slavery and its related problems."

However, he came to this view gradually, over the course of the Revolutionary War: "Apparently, in the war period Lafayette never questioned either the legitimacy or the morality of the institutions of slavery or the slave trade. Once he had even suggested to Washington that certain dangerous missions might be financed by the sale of slaves that he assumed could be captured."

But by the end of the war, Lafayette's view had changed. In a letter written in February 1783 to George Washington, Lafayette suggested that the two of them buy farmland to be worked by slaves, with the expressed purpose of gradually emancipating them. If the experiment worked, then perhaps they could spread emancipation throughout the rest of continental North America and the Caribbean:

"Now, My dear General, that You are Going to Enjoy some Ease and Quiet, Permit me to propose a plan to you Which Might Become Greatly Beneficial to the Black part of Mankind—Let us Unite in Purchasing a small Estate Where We May try the Experiment to free the Negroes, and Use them only as tenants—Such an Example as Yours Might Render it a General Practice, and if We succeed in America, I Will chearfully devote a part of My time to Render the Method fascionable in the West Indies—if it Be a Wild scheme, I Had Rather Be Mad that Way, than to Be thought Wise on the other tack."

Washington wrote back, and seemed to tentatively indulge Lafayette on the subject, but nothing was ever done about it. Lafayette did buy land in Guyana to try out this experiment, but he never followed through once his attention became focused on the French Revolution.

Tadeusz Kościuszko was opposed to slavery from the outset of the war. At one point during the American Revolution, he served in North Carolina under General Nathaniel Greene, where, as one biographer put it, Kościuszko "came face to face with the worst situations of human bondage, conditions he was not averse to criticizing" before his fellow officers.

For his war services, Congress granted him land in Ohio, and he also owned land in Maryland, in Washington D.C., and possibly in Virginia. It's unclear how much, if any, of these landholdings were ever cleared for cultivation, but it's fairly certain that none of them were being tended to by slaves.

After the war, in 1784, Kościuszko went back to Poland (actually the Polish-Lithuania Commonwealth). In Poland, there were political reforms going on that led to a new Constitution, which Kościuszko considered a European continuation of the American Revolution, i.e., a march toward freedom. In a letter in 1789, he wrote that he hoped that this progress "would not stop until all slaves, serfs, and oppressed peoples were treated as equals".

He then returned to the United States in 1797, whereupon he struck up a close friendship with Vice-President Thomas Jefferson, whom he saw "almost daily". He left the United States again in 1798, a little less than a year after he'd arrived, with his American estate in the care of others. Before he left, he wrote a will, appointing Thomas Jefferson the executor of his estate. In it, he requested that the profits from selling his land be used to free and educate as many slaves as possible, including the slaves belonging to Jefferson himself.

When Kościuszko died in 1817, however, Jefferson declined to fulfill his role as executor, and ultimately, none of the proceeds from the sale of Kościuszko's estate were ever used to free or educate any slaves. Jefferson cited his advancing age as a reason he wouldn't execute the will.

Baron Von Steuben supported slavery, at least to some degree, because he owned at least one slave. If it's any indication as to Von Steuben's thoughts on the social order, it was once suggested to him that perhaps he renounce his "baron" title of European noblity and become simply "Citizen Von Steuben." He laughed the suggestion off, saying that somebody else would get the title anyway, and then pointed out that Americans use similar titles, if informally. (A lot of Americans in colonial times were referred to by their former military or miltia titles, or professional titles. There were a lot of "Captains" and "Colonels" as well as "Judges", "Doctors", and "Squires".)

After the war, Von Steuben retired to an enormous estate in Oneida County, New York, where he owned some 16,000 acres. There, he owned at least one slave, because he freed him in his will:

"All the rest of my Effects I desire may be sold and after paying the expence of my Interment, be distrubuted amongst the Servants living with me at the time of my death—If the Negro Man Nathan is living with me at the time of my Decease he shall be then set at Liberty..."

The reference to "servants" suggests he likely owned more, that these were black slaves who were not freed upon Von Steuben's death, though I don't have a source for what was actually included in his estate at the time he died.

Casimir Pulaski didn't seem to leave any writings behind one way or the other on the subject of slavery. He almost certainly never owned any slaves, because he was living in Paris in 1775, and when he arrived in that city, he had no money. By early 1777, he had met the Marquis de Lafayette who recruited him to go to America and serve the Revolution. Pulaski went, and was killed in action in 1779, so he likely never had a chance to purchase any land in America. Thus, he almost definitely never owned any slaves, but what his actual thoughts on slavery were, are unknown.

Did Lafayette have any reaction to Napoleon's reintroduction of slavery after it was abolished in France?