I spoke too soon. OED has "word salad" in the classic reference, STEDMAN'S MEDICAL DICTIONARY (1915). It says there that "word salad" is from Forel (August Forel, 1848-1931?), so we can do earlier. The next citation is 1930. JSTOR includes some medical journals, but can do better than 1929. The MAKING OF AMERICA databases haven't been working all day. What's wrong?