In his commentary on a (possibly spurious) “protest” against Mozart attributed to Christoph Friedrich Bretzner over Mozart’s misuse of his play Belmonte und Constanze (Dokumente, 187), Deutsch writes: “In 1787 Bretzner anonymously published the novel Das Leben eines Lüderlichen [The Life of a Rake] in which Mozart’s Figaro is repeatedly mentioned and one of Cherubino’s two arias is quoted” (“Bretzner veröffentlichte 1787 anonym den Roman Das Leben eines Lüderlichen, worin Mozart’s Figaro wiederholt erwähnt und eine der beiden Arien Cherubinos zitiert ist”). As his source for this claim, Deutsch cites Greither (1970, 79). Deutsch’s claim has, in turn, been repeated by several subsequent writers, perhaps most prominently by Bauman (1987, 108). However, it seems that no writer has yet quoted any passage from Bretzner’s novel corresponding to the claim, and neither Deutsch nor Bauman gives direct citations from the novel.
Bretzner’s novel is, as the title suggests, based on Hogarth’s A Rake’s Progress (“lüderlich,” standard modern spelling “liederlich,” means “dissolute”; “ein Liederlicher” or “ein Lüderlicher” was a common translation of Hogarth’s “Rake” at the time.) Google Books includes scans of two different editions of Bretzner’s novel, both published in three volumes. The earlier edition was published anonymously in Berlin in 1787 (vols. 1 and 2) and 1788 (vol. 3). A second edition of the novel appeared under Bretzner’s name in 1790 (vol. 1), 1791 (vol. 2), and 1792 (vol. 3).
In Google Books, a search on the term “Figaro” in the three volumes of the 1787 edition turns up only one hit, the passage cited here. The reference to Figaro and “Cherubin” is omitted from the corresponding passage in the 1790 edition. The notion that Figaro is mentioned several times seems to derive from Greither.
A search of Google Books on the phrase “Mein Herz hat viel Leiden” reveals that it comes not from a translation of Mozart and Da Ponte’s opera, but rather from a translation of Beaumarchais’ play published in 1785: L[udwig] F[erdinand] Huber, Der tolle Tag oder Figaros Hochzeit, ein Lustspiel in fünf Aufzügen. Aus dem Französichen des Herrn von Beaumarchais nach der ächten Ausgabe übersezt. Dessau und Leipzig: Göschen, 1785, 36–37. The line “Mein Herz, mein Herz hat viel Leiden!” appears in Huber’s translation in every verse of the “Romanze” that Cherubin sings to the Countess, corresponding to “Voi che sapete” in the opera, but here based on Beaumarchais’ original, where the line is “Que mon cœur, que mon cœur a de peine." This phrase was, in fact, quoted quite frequently by other German writers in the ensuing decades.
However, Bretzner’s reference is clearly to the play (which was widely translated and performed in Germany), not to the opera, and thus Bretzner was not alluding to Mozart.