The great singer, Mabel Mercer, performed at Le Ruban Bleu in 1938. Here's what she has to say about EH:

She had a beautiful voice , not really a classical voice, but it had great tenderness, and when she sang those fados she was marvelous.

Virgil Thomson wrote about Elsie Houston a number of times while he was a columnist for the New York Herald Tribune (he was also a friend). A typical comment:

Miss Elsie is a great artist, a magnificent personality, and a practically unique mistress of vocal art.

And I think his take on her singing (which matches Harry Partch's conception to some extent) is on the mark. From a column following her Town Hall concert:

Few are the vocal artists whose range of color can compare with hers. Her musicianship is impeccable, her repertory distinguished, her voice agreeable, her diction tops. About the only thing she does not do well is to sing normally...Miss Houston does not vocalize classically, she vocalizes verbally...She is at her best when she evokes natural speech by musical sounds. These sounds are true musical sounds and her dramatic expression is true and easily communicated. Her whole performance is as practiced, masterful, and sophisticated as that of Lotte Lehman, for example. It is different, however, because it is built on a wholly different assumption...Carrying power, sheer vocal volume she throws out the window, sacrificing its advantages for the greater range of color and expressive range.

A reader and fellow Elsie Houston fan has sent me some clippings from the New York Herald Tribune that shed light on some matters EH.
First, the Tribune obit (unlike the NYT) hints at the contents of the suicide notes: "she was upset about financial matters."
Second, her father was indeed James Frank Houston (who was apparently still alive and practicing dentistry into the 1940s) whereas her mother was "a Brazilian whose family emigrated to Rio de Janeiro from Portugal 300 years ago."
Third, a long profile by William G. King in the New York Sun (March 30, 1940) repeats a claim I've read elsewhere that she sang in the first Paris performance of Manuel de Falla's "El Retablo de Maese Pedro." If this is true, she would have been 20 years old and still a student of Lilli Lehman. It is more likely she sang at a later performance while she was a student of Ninon Vallin (a Falla favorite).

I'm heading out to the Boston Public Library tonight to see what else I can glean from the other New York papers.

Some additions to the Elsie Houston in New York chronology, including some notes from Lotte Lenya in letters to Kurt Weill about her experience performing in Le Ruban Bleu in 1938.