As Mac Mack McCormick's research on Johnson appears to be be locked away permanently I think that Wardlow is about the only hope of getting more.

Incidentally Some of McCormick's work in which he collaborated with Paul Oliver has finally after a long, long wait just come to light with "The Blues Come to Texas" published by Texas A & M University Press. A hefty tome with much detail on the blues in Texas. I am just about half way through it's 457 jam packed pages.
My only petty niggling disappointmenr so far is that in a photograph of Paul Oliver with two musicians they are incorrectly shown to be Big Bill Broonzy and Brownie McGhee in 1958. In fact they are Big Bill Broonzy and Brother John Sellers in the year 1957 when Broonzy and Sellers were on tour together in the UK.