I've found more mid-20th-Century uses of 'scientism'.
First, there is this passage from Emil Brunner's Gifford Lectures (1947-48):
Science and art serve men best if they remain true to their own laws. They must be 'autonomous'. But if this autonomy is ultimately final it cannot but degenerate into sterile inhuman intellectualist 'scientism' and into l’art pour l’art aestheticism. If however their autonomy is understood as theonomy they keep their independence and yet are united to natural life and ethical principles by a unity standing above all of them.It looks like Brunner dropped 'scientism' from the book that was drawn from his Gifford Lectures. The book in question is Christianity and Civilisation (Charles Scribner's Sons, 1948). Interestingly, the latter portions (at least) of that book (beginning at p. 85) echo some of the themes connected with other mid-20-Century uses of 'scientism'. For instance, Brunner mentions Comte and Spencer several times and says that liberalism degenerates and leads to collectivist tyranny (pp. 139-140). Brunner discusses fascist and communist forms of totalitarianism and sees both as threats to individualism and freedom. It was in that context that he had (in his lectures) used the word 'scientism'.
'Scientism' appears once in C. Wright Mills' 1959 book The Sociological Imagination:
In the meantime, philosophers who speak in the name of science often transform it into ‘scientism,’ making out its experience to be identical to human experience, and claiming that only by its method can the problems of life be solved. With all this cultural workmen have come to feel that ‘science’ is a false and pretentious Messiah. (Mills, The Sociological Imagination [Oxford University Press,1959], p. 16)Finally, the mid-20th-Century, Cold War linkage of scientism and Marxism resurfaces in this 1992 piece by Vaclav Havel. In it, Havel says,
The modern era has been dominated by the culminating belief, expressed in different forms, that the world -- and Being as such -- is a wholly knowable system governed by a finite number of universal laws that man can grasp and rationally direct for his own benefit. This era, beginning in the Renaissance and developing from the Enlightenment to socialism, from positivism to scientism, from the Industrial Revolution to the information revolution, was .... an era of systems, institutions, mechanisms and statistical averages. It was an era of ideologies, doctrines, interpretations of reality, an era in which the goal was to find a universal theory of the world, and thus a universal key to unlock its prosperity. Communism was the perverse extreme of this trend. (Havel, New York Times [March 1, 1992])