I am editing a manuscript that mimics the style of late 19th-century British writing. To what extent were serial commas used during this time period?
I would venture to say, yes. (Just for the sake of completeness in this answer, I am considering the Victorian era to be from 1837 to 1901.)
In this article about the Oxford comma, which is another name for the serial comma, the author gives two examples showing punctuation from the same British periodical, but 125 years apart:
Perhaps this is better conveyed by way of example – see which of the two looks better to you now:
The Boxing day audience assembled to witness Lord Harris's team play their first match in Victoria numbered 10,000 at least. The weather was fine, and the wickets in magnificent run getting order, as may be imaged from the fact that when, after three days of cricket, the match was drawn, there had been only three innings played for the 858 runs scored. The Victorians won choice, and began the batting. Four wickets had fallen for 51 runs, when Mr. Donald Campbell (of the Oxford University Elevens of 1874, '75, and '76) commenced his finely hit three figure innings of 128 runs – an innings that one journalist stated was "played without a chance," and another that it "comprised hits for thirteen 4's and eleven 3's." Mr. Campbell was eleventh man out, the score at 279; he was enthusiastically cheered by all on the ground. That day's cricket ended with the Victorians having lost 12 wickets and scored 288 runs.
Seventeen wickets fell on the first day, though pitch liaison officer Raman Subba Row gave an old strip qualified approval. Taylor decided to bat but was one of four early wickets, three lbw. Gloucestershire limped to their lowest Championship total of 2004 before Averis dismissed the Sussex top four. The ball still swung and seamed after a blank second day: last pair Kirtley and Lewry claimed a slender 21-run lead, with 50 in eight overs, and were denied a batting point only by a catch to tell your grandchildren about - Hussey diving full stretch in the deep after a long chase. Spearman showed the pitch could be mastered, with seven fours and a six in his eighth half-century of the season. Gloucestershire resisted setting a target; flirting with relegation, they wanted to be sure of a draw and also faced penalty points for a slow over-rate - a regulation applying only once they had bowled for four hours or more in the match. So they batted on until lashing rain ended play on the final afternoon.
Both these are unremarkable pieces in themselves: they were chosen for this reason. But they were also chosen because they were from the same annual publication, Wisden's Cricketers' Almanack. The first passage is from the 1880 edition, the second is from the 2005 edition. But the way they are punctuated is significantly different. The second excerpt is slightly longer and contains 20 pieces of punctuation. Noticeably this includes two dashes - punctuation marks that were certainly used in the past, but which have become much much more popular in recent years. The first passage contains 34 pieces of punctuation.
And you will notice, that the first excerpt from 1880 contains a serial comma (which I bolded).