With the help of Walter Fanta, I have found a section in Musil's notebooks where he seems to dismiss Ludwig Hohl (whose name in German means hollow, or empty) in one short sentence:"Swiss aphorist: Now at least I know what an air-head [Hohl-Kopf] is!"
Of course, it remains unclear how much he had read of Hohl upon making this pronouncement. He rejected Proust, after having read (by his own account) no more than 10 pages of his work. While Musil was often generous to young writers (Walser and Kafka, for example), he was notoriously unfriendly to successful contemporaries. Hohl would have fit, rather, in the category of under-appreciated younger author in need of Musil's aid. Perhaps he simply could not resist the pun; or, indeed, for the man who proposes that to have no fixed qualities is at least partially a positive state of openness, having an airy, open, or hollow head might actually be a good thing.