The problem was, I've never been able to find a likely 'Helen Beauclaire' via Google or any other search engine, so I was beginning to think she was a bit of a myth. But last night, I struck gold.
It turns out that her surname was not Beauclaire, but Beauclerk — a homonym, but not one that Google is going to notice. And Googling on Helen Beauclerk worked: various online booksellers have her books. It may help that she was the translator of some of Colette's work into English, and that she was married .. well, OK, not legally married, but effectively so ... to the artist Edmund Dulac, who illustrated some of her work.
Oh, and she's mentioned in the Encyclopaedia of Fantasy, apparently.