I already knew, from his incomplete story The Dark Tower that Lewis had read JW Dunne - and this was also an interest of Tolkien's, and probably the subject of Inklings discussion.
I have found further evidence of critical engagement with JW Dunne's ideas, and of Lewis's interest in Time in a collection of short memoirs and pieces about Lewis, recommended me by commenter Dale (thanks).
The discussion described below was more than a decade after the Inklings Thursday evening meetings had ceased - indicating a prolonged interest in Time on the part of Lewis, even after he had ceased to meet Tolkien regularly.
From We Remember CS Lewis edited by David Graham, 2001.
From the chapter by Daniel Morris - Encounter in a Two-Bit Pub. Describing a conversation between Graham and Lewis in 1959. This passage derives from a letter written shortly after the conversation.
Then he asked me what I was doing in England. Thence to mathematics, biochemistry, and the fourth dimension.
He was much interested in the latter, and wanted to know if I knew of Hinton's ideas, including the one that with enough practice you can actually visualize the fourth dimension.
I said that with all my practice, I can work with the figures easily enough, but not visualize them - it can't be done.
He was insisting that the whole idea is pure imagination (he's read Hinton and Dunne and Ouspenski and Abbot) like the square root of minus one. And I wasn't willing to make it that imaginary, considering curvature of space, for example, which seems to be experimentally true - and meaningless, unless the universe really is four dimensional.
He went into Dunne a good bit (that is, JW Dunne: An experiment in time, published in 1925) and he doesn't see (neither do I) why Dunne had to postulate an infinity of times at right angles to one another. Two times would cover he whole thing. Granted, that leaves a mystery as to what makes the thing run, but Dunne simply puts that off at infinity.
And thence to previsions, and to extrasensory perception, and predestination (...)
In the course of our talk about Dunne, and such, he said it was a shame we couldn't control the rate of flow of time.
As it was, the clock was rapidly moving on towards half past seven, and the end of this delightful talk he was having with me.