The basic problem with this question is that the Papacy really didn't start producing a large number of encyclicals until Leo XII in the middle of the 19th century. Prior to that, the Papal Bull would be used, as would apostolic letters, but Leo XIII really revolutionized the practice. I highly recommend Papalencyclicals.net for more sources.
Another problem is that slavery and the later abuses come predominantly from areas under the British crown. The French had lost most of their oversees holdings by the middle of the 18th century, the Spanish and Portugese had outlawed a majority of slavery by the end of the 17th century. As far as I know, the only major involvement after this was during the opium war in China (which is well past Catholic influence).
A third problem is "official position" and "official efforts" are often far removed from actual realizations. Spain and Portugal outlawed slavery, but it still continued. For that matter, the US outlawed the import of slaves, but it continued until slavery was abolished.
The big list for slavery timeline can be found here. Any involvement by the Holy Roman Emperor may be considered "action by the Church" depending on your point of view, but even still, there does not seem to be terribly much unilateral action.
- Enslavement of natives condemned in the 16th century (note: the two major players in the enslavement of natives were the Portugese and the Spanish, both of whom had gotten rid of those practices within about 150 years (4 generations or so) of the condemnation)
- Direct condemnation of all slavery is found here, but that is in the 19th century.
For the (mid/late) 19th century, Rerum Novarum is arguably the single most important document in Catholic Social Teaching since the Bible itself (actually, when I was in grad school, it was considered the "first, best place" to learn the Church's opinons re: social justice). If you like that, there are at least a dozen encyclicals based off of that, some being as clearly named as "Forty years later" and "One hundred years later", but those are from the beginning of the 20th century onward.
Immediately, this document on almsgiving from the Council of Trent comes to mind.
The pastor, therefore, should encourage the faithful to be willing and anxious to assist those who have to depend on charity, and should make them realise the great necessity of giving alms and of being really and practically liberal to the poor, by reminding them that on the last day God will condemn and consign to eternal fires those who have omitted and neglected the duty of almsgiving, while on the contrary He will praise and introduce into His heavenly country those who have exercised mercy towards the poor.
Then, there is Clement XIII's "On Unity Among Christians"
Among the fruits of justice, mercy to the poor should certainly be considered the most important.