Hal--

Well, as I recall, Sundquist's connection is by analogy, not direct.  If so,
then the connection is to be made, not ready-made.

Maybe I can help a bit, but if this is stuff you've already checked out I'm
sorry for the needless duplication.

Two books that come to mind are Otto Olsen's seminal Thin Disguise which is
about the Plessy v Ferg. Case (as you say, Tourgee was a lawyer; he went
south during Reconstruction, to NC from NY, and was there the whole time,
till 1877.  Became a judge and was pretty well accepted, considering he was
a carpetbagger.  Then went back north [to Ohio, as I recall] and as a
Radical Republican began to write Reconstruction novels--Fool's Errand and
Bricks Without Straw being his best known, along with a history of the KKK
[The Invisible Empire, I think it was titled] which he published separately
and as an attachment to the 2nd ed of F's E in 1880.  In the early 90s he
was called in on the Homer Plessy case in N. Orleans and in time became the
lead atty for Plessy when the case got to the Supreme Ct in '95]).  I'm
operating from memory, so maybe I'm off a bit on this or that detail.
Probably you know this much already.

Twain's PWilson, '94, was written in Italy, so what MT knew directly about
the Louisiana background of Plessy and Tourgee is, I think, conjectural.
Same for what MT knew about ATourgee before Plessy, when AT was a novelist.
However, Gribben's MT's Library says that MT owned both Bricks and Inv Emp.
(1880 for both).

Anyway, Olsen's study is fine and includes, of course, a good deal about
Tourgee in his analysis of the Plessy case. I can't say I recall that
Tourgee's religion was highlighted, however.  In truth, I can't recall a
religions emphasis in ANY critical/biographical scholarship on Tourgee, but
maybe I missed it.

You can also read Tourgee's brief in the Plessy case, filed with the Sup.
Ct. (Landmark Briefs reprinted it, among others).  I don't recall any
religious emphasis there either, but again, maybe I am not recalling
accurately.

Then there's a critical bio in the Twayne series--by Theo Gross, I
believe--several years ago, probably the 70s.

Hope this offers a little bit that's new and is helpful.

Terry