Question: Which Catholic priests were given diplomatic missions?
Are there prior cases, (before 1822) of Catholic priests tasked to travel on secular diplomatic or administrative missions?
The Catholic Church has been active in international diplomacy for a very long time. Instances of Catholic Priests performing diplomatic missions (secular and otherwise) are thus too numerous to count and date back to the very beginning of what we might recognize as modern diplomacy.
The Catholic Church is not just a Religion but has been temporal earthly state since medieval times. An earthly state which at times had great power and riches to rival other nations in Europe. The Donation of Pepin (756AD) which conveyed upon the Church the lands which contained the Vatican States negotiated by Vatican emissaries between the Lombard's, Francs and Church with the aid of a forged Imperial Roman Decree The Donation of Constantine. Since that time the church shared interests with other temporal kingdoms, as well of coarse as religious.
Even before the Church had temporal concerns as any great kingdom it conducted international diplomacy as a way of both looking after it's interests and growing it's power. It thus pioneered and coined terms which are still in use by diplomats today.
Diplomacy of the Roman Catholic Church
From the 6th century, both legates and (lesser-ranking) nuncii (messengers) carried letters of credence to assure the rulers to whom they were accredited of the extent of their authority as agents of the pope, a practice later adopted for lay envoys. A nuncius (English: nuncio) was a messenger who represented and acted legally for the pope; nuncii could negotiate draft agreements but could not commit the pope without referral. In time, the terms legate and nuncius came to be used for the diplomatic representatives of secular rulers as well as the pope. By the 12th century the secular use of nuncii as diplomatic agents was commonplace.
The Catholic Church has maintained a 4 year post-graduate school for diplomats Pontifical Ecclesiastical Academy since 1700 an institution to train Catholic Priests in Diplomacy. Today it's used to train priests to fill it's more than 100 diplomatic presences around the world which the church maintains including a seat at the United Nations (permanent observer status, not a voting member) and Council of Europe. But Catholic priest involvement in "secular" diplomacy go back further to the creation of the church.
Arguable the church itself was invented / created / unified through the first church council at Nicea where the Roman Emperor Constantine who was then not yet Christian, called all the bishops of the church to him in the town of Nicea a suburb of Constantinople in modern day Turkey. This arguable was the first instance of a Papal diplomatic mission as the Pope decided not to attend but instead send his representative.
From the Fall of the Western Roman Empire and the schism between Byzantine Empire the Catholic Church's emissaries and diplomats were all over Europe. Later in the medieval times when the Church became a global concern it's diplomats perhaps were the only and most prevalent infrastructure which connected diverse empires, kingdoms and civilizations.
Two of my favorite Examples of Papal diplomacy, but by no means the earliest would be..
The diplomatic mission sent to the Mongol's Güyük Khan in 1245 by Pope Innocent IV.
The mongols had just defeated the kingdoms of Poland and Hungary and opened up central Europe to their raids and the Pope was reaching out to discern their intentions. The Pope dispatched priests with gifts and his message to the Khan and the Khan's chilling response which they carried back to Rome, still resides at the Vatican. More of a ceramic plate than a letter actually.
Mongol-Papal Encounter: Letter Exchange between Pope Innocent IV and Güyük Khan in 1245-1246
My second example would be the Treaty of Nerchinsk (1685) which delineated the boarders between the Russian Empire under Peter the Great and Qing Empire, ruled by the Emperor Kangxi. What is so interesting about this treaty and the involvement of the Church was that neither court had a way of speaking with the other side. Nobody spoke both languages. Priests were employed because they spoke the local dialect and a common language of Latin, which enabled the conversation which lead to the treaty.
Other examples of Priests carrying out important diplomatic work would include.
De Anza Trail, 1775 and what became a 1200 mile network of outposts for the kingdom of Spain. What do you do if you are the king of Spain with vast possessions in the North and Central America's with no way to keep track of it? You send in Priests to build missionaries and make contact with the natives and use the network for intelligence on your lands.
The Jesuits In Japan(1540s), They brought Catholicism to ancient Japan increasing the reach of the church, spreading to more than 100,000 members, before incurring a crack down and retribution by the Japanese emperor.