I am sure Mallow was based on a real person (as most of Jones' characters are). There was just too much detail about the IWW (Wobblies) for Jones not to know such a person. Also since Jones was only in his 20s when he wrote FHTE I don't think he would have known enough about the Wobblies on his own to completely invent such a character. Most likely, Jones did meet an ex-Wobbly much like Jack Malloy. I would also like to know which real-life person Jones based Sgt. Warden on.
BTW, Robert E. Lee Prewitt was based on an Army buddy of Jones' named Robert Stewart.