After months of research (not on this question!) I've found the following concerning why Dickinson Miller published his famous paper on free will under a pseudonym:
'Hume Without Scepticism' and the article on 'Freewill as Involving Determination' both appeared in Mind under the name R. E. Hobart, though Miller openly acknowledged his authorship in footnotes of subsequent articles on 'James and Analysis' and the knowledge-problem. Further, he travelled to England, Ireland, and Italy from 1932 to 1934 with a passport issued in Frankfurt/Main under the name of Richard Emlen Hobart, the first name being derived from his nickname and the last two from family names. He even corresponded with his brother about a permanent legal change of name but abandoned 'Hobart' on returning to America in 1934. His specific motives for adopting the name of Hobart are obscrue because he had an intense sense of privacy and vigilantly guarded it. He did, however, tell one associate that he thought people who knew him would not read his articles because his links with James would make him appear "too old". To another he explained that it is perfectly appropriate to use any name that suites one's private convenience. The content of the articles may have been a factor. His main points in the article on Hume had been dismissed by an eminent British scholar as insubstantial, not worthy of publication.
(From Loyd Easton's introduction to a collection of Miller's essays, Philosophical Analysis and Human Welfare [Reidel, 1975].)