Positive connotation of "fluke"?

The OP said:

To me, the best proof in support would be an example of the broader use by a first-rate writer

Here is an example of fluke being used, first as an unlucky accident, and second, as a lucky accident. The quotation is perhaps too long, but it illustrates that the first speaker used fluke as unlucky, and the second as lucky. From E. Phillips Oppenheim, The Vanished Messenger

"But how on earth did you get to know about me," Mr. Dunster asked, "and my errand? You couldn't possibly have got me here in an ordinary way. It was an entire fluke."

"There, you speak with some show of reason. I have a nephew whom you have met, who is devoted to me."

"Mr. Gerald Fentolin," Mr. Dunster remarked drily.

"Precisely," Mr. Fentolin declared. "Well, I admit frankly the truth of what you say. Your - shall we say capture, was by way of being a gigantic fluke. My nephew's instructions simply were to travel down by the train to Harwich with you, to endeavour to make your acquaintance, to follow you on to your destination, and, if any chance to do so occurred, to relieve you of your pocket-book. That, however, I never ventured to expect. What really happened was, as you have yourself suggested, almost in the nature of a miracle. My nephew showed himself to be possessed of gifts which were a revelation to me. He not only succeeded in travelling with you by the special train, but after its wreck he was clever enough to bring you here, instead of delivering you over to the mercies of a village doctor. I really cannot find words to express my appreciation of my nephew's conduct."

"I could," Mr. Dunster muttered, "very easily!"

As for the neutral meaning of fluke, I cite from The Free Dictionary

A chance occurrence: That spring snowstorm was a total fluke

(There are other dictionary definitions of fluke that support a neutral or negative meaning, but this answer is too long already.)

As far as I know, none of the authors of the following examples are considered to be "first-rate" by anyone (which helps make whatever "evidence" that's contained below fall well short of the "best proof" that you seek), but I’m most familiar with hearing and seeing “fluke” used negatively when the subject is disappointing performances, grades, or test results that might be the result of accidental aberrations or anomalies, such as:

I was tempted to offer a second interview to see if her poor performance was a fluke.

(from Crazy Good Interviewing: How Acting A Little Crazy Can Get You The Job by John B. Molidor [with emphasis added], via ‘Google Books’)

or

[I’d] review the reports very carefully to see if there was a problem or if the results were a fluke.

(from Finding the Best and the Brightest: A Guide to Recruiting, Selecting, and Retaining Effective Leaders by Peg Thoms [with emphasis added], via ‘Google Books’)

or

[W]hen I got a D last week … I thought it was a fluke, but now I have to admit that I’m floundering here.

(paraphrased, with emphasis added, from Trapped: Caught in a Lie by Melody Carlson, via ‘Google Books’)

"We are quick to dismiss such results as the statistical flukes they almost certainly are"

--Peter W. Huber, Hard Green: Saving The Environment from the Environmentalists

He's a good writer; no further