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In attributive use, the phrase penny-a-line originally and derogatorily referred to a journalist who was paid at the rate of a penny a line.—Cf., below, the explanations in the 1823 slang dictionary.

The following, for example, is from Which Words?, published in the Daily Post (Liverpool, Merseyside, England) of Friday 11^th October 1985 [page 16, column 1]:

A many-splendoured word is hack. […]
[…]
In American campus use hack is to vomit, in Standard English to cough. Of course, it once meant a penny-a-line journalist, but it has softened into an affectionate tag for a fellow journalist, especially one jointly suffering, as in war reporting, in a temporary cameraderie [sic].

The phrase penny-a-line came to be also used attributively of low-quality writing.—Cf., below, “penny a line flourishes” in the quotation from the London Mercury (London, England) of Saturday 21^st October 1826.

—Cf. also ‘penny-a-liar’: meaning and origin.

The first two mentions of payments at the rate of a penny a line that I have found are as follows:

1-: From The British Press (London, England) of Monday 2^nd July 1804 [page 2, column 3]—here, however, penny a line is positively connoted:

The articles in The Oracle furnish internal evidence that they are not from the pen of the Treasurer of the Navy. Mr. Canning is a gentleman of literary talents, of which the writing in The Oracle bears no mark. It is impossible that he should degrade his character and mispend [sic] his time by writings which would be executed for a penny a line in a superior style and manner by the humblest agent in any newspaper office in London.

2-: From Vicissitudes in Early Life; Or, The History of Frank Neville (London: C. Chapple, [1807]), by Dennis Lawler [volume 1, chapter 6, page 81]:

A man of first-rate Classical acquirements, can look for no better fortune, during his whole life, than the drudgery of teaching under another, or writing paragraphs for the newspapers at a penny a line.

The earliest attributive uses of the phrase penny-a-line that I have found are as follows, in chronological order:

1-: From an account of the address from the inhabitants of Bethnal Green to the Queen, which took place on Wednesday 30^th August 1820, published in The Imperial Weekly Gazette, and Westminster Journal (London, England) of Saturday 2^nd September 1820 [page 2, column 1]:

The Courier, in describing the procession, says, with its usual venom, “It was preceded by eight ‘mutes’ on horseback to clear the way; that the carriages, for the most part, were drawn by undertaker’s horses [&c.]”.
The fact is, the Courier had planted at the door of their office, the whole corps belonging to their establishment, consisting of compositors, pressmen, fly boys, (alias printers’ devils,) and two or three penny-a-line gentlemen, to give an account of the procession, and from the report of this horde, they have fulminated the nonsense that appeared on Thursday.

2-: From Slang. A Dictionary of the Turf, the Ring, the Chase, the Pit, of Bon-ton, and the Varieties of Life, Forming the Completest and Most Authentic Lexicon Balatronicum Hitherto Offered to the Notice of the Sporting World (London: Printed for T. Hughes, 1823), by the British author and journalist Jon Bee (John Badcock – fl. 1810-1830) [page 183]—here, qu. is the abbreviation of quasi:

Two-penny men—i.e. two-pence per line for fabricating articles of intelligence for the newspapers, paid to men calling themselves reporters [qu. porterers? carriers] forsooth. When the type used for such minor purposes ceases to be minion (e mignon, little) and devolves into bourgeois (or commonalty letter)—into brevier, or short letter—then three half-pence is the dignè payment per line. Sometimes called ‘penny-a-line-men;’ but this applies only to such as work at under price—dungs; an invidious kind of generalising.

3-: From The Australian (Sydney, New South Wales, Australia) of Saturday 15^th July 1826 [page 2, column 3]:

As to “general principle of character,” and also as to education, Mister Taylor is on a par with many, and superior to many others of the gentlemen reporters who frequent the Police Offices in the great metropolis of London. His character, bad as it may be, will bear a scrutiny with nine tenths of the penny-a-line youths, who feed the Newspapers of London with nine tenths of their food, out of Parliamentary seasons.

4-: From a transcript of remarks made by Mr. Scarlett, a counsel, in an action for libel, in the Court of King’s Bench, on Friday 20^th October 1826, published in the London Mercury (London, England) of Saturday 21^st October 1826 [page 4, column 2]:

The learned Counsel wished the editors of newspapers would pay their Reporters by the merit, and not by the quantity of their communications. For himself, he scarcely ever looked into the Police reports of a paper; for, before to-day, he had heard of them being penny a line flourishes!

5-: From The London Press Gang, published in the Weekly Dispatch (London, England) of Sunday 22^nd October 1826 [page 340, column 2]:

The readers of the dailies may rest assured, that in Police Reports, the laughable and pathetic descriptions of the bye-play and mannerisms of the witnesses, or the accused, are, in nine cases out of ten, sheer humbug and invention.—Every person of the least discernment must have already discovered how slight are the pretentions of most of these penny-a-line men—for that is the pay they receive from several of the papers—to a knowledge of the principles either of the English law or language.

6-: From the Public Ledger, and Daily Advertiser (London, England) of Saturday 20^th January 1827 [page 1, column 5]:

MEETING OF MIDDLESEX MAGISTRATES.

A meeting of the above Magistrates took place on Thursday, at the Sessions House, Clerkenwell, wherein it was proposed that every Court held for public business, be an open Court […].
[…]
Mr. Flower was decidedly against the admission of the Public. He thought no benefit could arise from the Gentlemen of the press, the penny-a-line Gentlemen—Laughter […].