So, thanks to David B. for pointing out to me that the full letter I quoted from in my post on Dalfonzo's book DOROTHY AND JACK can be found in Lewis's COLLECTED LETTERS (Vol III p. 689-690). The letter was written on 22/12/55 (that is, December 12th 1955). The bulk of the letter contrasts 'clerks' (who I take him to mean the literati, self-appointed judges) with those who create popular art; he much prefers the latter.
The line that interests me comes in the paragraph that reads in full:
"I hadn't really thought about it before, but of course Tolkien's females are as you describe them. And one couldn't, from internal evidence, diagnose the cause, because so many causes are equally possible."
Hooper's note reads simply "It is not known how Savers described them."
So that adds to the context without, unfortunately, providing the information we most wanted.
The one takeaway for me is that from this we definitely know that D.L.S. read Tolkien -- which might have been well known to Sayers scholars but was news to me, and welcome news at that.
--current reading; another volume of the same light novel, more on Lindop's Wms*
*C.W. has just met the love of his life. His wife doesn't know about her yet.