Author here - I love Dune and mostly just used the phrase for the title. But I did write this about Herbert and the drug/spice trade which might be of interest: https://lareviewofbooks.org/article/pharmacology/

Yes, quite contrary to my expectations. The phrase isn't even used outside of the title. I always knew it only as a Dune reference; was the actual spice trade ever described in such terms?

Pretty sure the answer is no.

all I know is they better keep that black pepper flowing across the ocean to my plate or war could break out

Ah yes, hundreds of years of global spice trade and colonisation for the sake of spices, yet the flavors that stuck were salt and pepper. I am so disappointed.

The phrase in the title is also not from Frank Herbert. It is from Lynch's movie adaptation of the books.

Sure, but "The spice must flow" specifically as a slogan is an iconic phrase original to the movie.

It would be nice if it could be surreptitiously slipped into the new one somehow, as a tip of the hat to Lynch.

Reminds me of the phrase "Luke, I am your father", which is not actually used in the movie.

I've read all of the Herbert's Dune before watching Lynch movie, cited phrase above stood out to me at the time even when embedded within the broader context of a dialogue. Its possible though that I've picked it up from someone who picked it also up from the movie, sure.