The Coward citation from 1929 is also pretty clearly in the sense of homosexual. The “green carnation” is s clear reference to Oscar Wilde.
1929 N. Coward (We all wore) Green Carnation in B. Day N. Coward: Compl. Lyrics (1998) 114/3 Art is our inspiration, And as we are the reason for the ‘Nineties’ being gay, We all wear a green carnation.
The Bringing Up Baby citation from 1938 is not a clear use in this sense. Ron Butters makes a persuasive case that this is not a use of the slang sense of homosexual. (I have to update the Big List entry on this one.)
The candidate citations that I’ve seen that predate 1929 are almost certainly not used in this particular slang sense, although they often seem so on the surface. The problem is that in the opening decades of the twentieth century gay had a number of sexual or hedonistic senses, and determining exactly what was meant in a particular case is often impossible. That’s probably why it made such a great code word for the homosexual community; it provided plausible deniability if overheard or directed at the wrong person.
(My use of homosexual here sounds really bad. Normally I’d write things like “the gay community,” but I’m trying to avoid using the word gay to avoid confusion.)