Do the teachings of Catholic Church prohibit the use of “Amen” for purposes other than prayers?
The short answer is no.
First of all, Our Lord employed Amen outside of prayer.
- Amen, amen I say unto you: He that believeth in me, hath everlasting life." - (John 6:47 Douay-Rheims)
Jesus frequently said "Amen" ("Truly") to preface a statement (as compared to saying "Amen" at the end). When Christ is leading off with amen, it not only implies that what follows is true but also that the person making the statement has firsthand knowledge and authority about it".
In the Gospels of Matthew, Mark, and Luke, Jesus used a single Amen (Truly, Verily) to introduce over 50 statements of truth (Ref. 5 below, Strong's Exhaustive Concordance). Consider these references: Matthew 5:18, Matthew 18:12-13, Matthew 25:11-12, Mark 9:41, Mark 11:23, Luke 18:17. To the criminal on the cross who asked Jesus to remember him, Jesus said, "Truly I say to you, today you shall be with Me in Paradise" (Luke 23:43).
When Jesus says "Truly, Truly" (double Amen) at the beginning of a statement, he is telling us that the following words are extremely important. When Jesus begins a teaching and says, "Amen, amen, I say to you," our listening ears should be fine-tuned to take note instantly of what our Lord is going to say, for it is of the utmost importance.
Finally, the Catholic Encyclopedia explain the use of the word amen as follows:
I. In the Holy Scripture it appears almost invariably as an adverb, and its primary use is to indicate that the speaker adopts for his own what has already been said by another. Thus in Jeremiah 28:6, the prophet represents himself as answering to Hananias's prophecy of happier days; "Amen, the Lord perform the words which thou hast prophesied". And in the imprecations of Deuteronomy 27:14 sqq. we read, for example: "Cursed be he that honoureth not his father and mother, and all the people shall say Amen". From this, some liturgical use of the word appears to have developed long before the coming of Jesus Christ. Thus we may compare 1 Chronicles 16:36, "Blessed be the Lord God of Israel from eternity; and let the people say Amen and a hymn to God", with Psalm 105:48, "Blessed be the Lord, the God of Israel from everlasting: and let all the people say: so be it"/(cf. also Nehemiah 8:6), these last words in the Septuagint being represented by genoito, genoito, and in the Vulgate, which follows the Septuagint by fiat, fiat; but the Massoretic text gives "Amen, Alleluia". Talmudic tradition tells us that Amen was not said in the Temple, but only in the synagogues (cf. Edersheim, The Temple, p. 127), but by this we probably ought to understand not that the saying Amen was forbidden in the Temple, but only that the response of the congregation, being delayed until the end for fear of interrupting the exceptional solemnity of the rite, demanded a more extensive and impressive formula than a simple Amen. The familiarity of the usage of saying Amen at the end of all prayers, even before the Christian era, is evidenced by Tobit 9:12.
II. A second use of Amen most common in the New Testament, but not quite unknown in the Old, has no reference to the words of any other person, but is simply a form of affirmation or confirmation of the speaker's own thought, sometimes introducing it, sometimes following it. Its employment as an introductory formula seems to be peculiar to the speeches of Our Saviour recorded in the Gospels, and it is noteworthy that, while in the Synoptists one Amen is used, in St. John the word is invariably doubled. (Cf. the double Amen of conclusion in Numbers 5:22, etc.) In the Catholic (i.e. the Reims) translation of the Gospels, the Hebrew word is for the most part retained, but in the Protestant "Authorized Version" it is rendered by "Verily". When Amen is thus used by Our Lord to introduce a statement He seems especially to make a demand upon the faith of His hearers in His word or in His power; e.g. John 8:58, "Amen, Amen, I say unto you, before Abraham was made, I am". In other parts of the New Testament, especially in the Epistles of St. Paul, Amen usually concludes a prayer or a doxology, e.g. Romans 11:36, "To Him be glory for ever. Amen." We also find it sometimes attached to blessings, e.g. Romans 15:33, "Now the God of peace be with you all. Amen"; but this usage is much rarer, and in many apparent instances, e.g. all those appealed to by Abbot Cabrol, the Amen is really a later interpolation.
III. Lastly the common practice of concluding any discourse or chapter of a subject with a doxology ending in Amen seems to have led to a third distinctive use of the word in which it appears as nothing more than a formula of conclusion — finis. In the best Greek codices the book of Tobias ends in this way with Amen, and the Vulgate gives it at the end of St. Luke's Gospel. This seems to be the best explanation of Apocalypse 3:14: "These things saith the Amen, the faithful and true witness who is the beginning of the creation of God". The Amen who is also the beginning would thus suggest much the same idea as "I am Alpha and Omega" of Apocalypse 1:5, or "The first and the last" of Apocalypse 2:8.
There you have it in a nutshell. Amen for that!