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.)