I've discovered a character called Sara Monday in a novel called Herself Surprised by Joyce Carey (1941). But nothing I've read so far indicates who the 'louche genius' was. I'll keep looking!
Apparently Sara Monday is the lover for a time of an artist called Gulley Jimson, for whom I assume she also acted as a model. It's all good enough for me!
Brilliant. I read Cary's The Horse's Mouth when I was a student, and Jimson was certainly a louche genius. I don't remember Sara Monday, but Googling indicates that she fits the bill as model friend (and ex-wife) in the series of novels. Thanks for the answer.