L. Frank Baum wasn't the only one to work for Reilly & Britton under a pseudonym. John R. Neill illustrated at least one book under the name Ivin Ney - as to why, I really can't imagine! A Knight of the Wilderness was published in 1909, and contains 6 color plates by Neill under the assumed name. The book deals with Abraham Lincoln's years in Illinois. After a little thought, the pseudonym does make some sense - it's a kind of Russian-ized version of his real name (Ivin/John, Ney/Neill).
I don't know of any other examples of Neill working under this alias. 1909 was a busy year for him, with The Road to Oz and the start of the Neill Gift Book series of poetry books. Maybe Reilly & Britton was worried about too much exposure in various genres for their popular Oz artist. The ink and wash style of the illustrations for A Knight of the Wilderness is very much in line with the poetry books or Neill's magazine work of the time, as opposed to his Oz drawings.