All, I am actually a Cross researcher, but may be able to provide a useful link for this group.
I've browsed the Ishmail mailing list, board, and Ancestry site & have seen several Ishmael sources tempted by the 1805 Greene County marriage record between Benjamin Essman and Darkis Cross, surety provided by Shadrack Cross. I believe those researchers are right to be intrigued. "Darkis" Cross was Shadrack Cross's sister & they were children of Henry Cross & Lydia Morris of Greene County.
My reasons for thinking Benjamin Essman was probably Benjamin Ishmael are:
1) in the 1820 census, Henry Cross, then aged about 79, is listed as head of household adjacent to Benjamin Ishmael in Giles County, Tenn. It is a large household with many youngsters. Henry's son Shadrack had six sons by two wives, all born by 1820 and since he doesn't appear elsewhere in the census, I believe he and his family, and possibly brother Benjamin F. Cross & family are included in Henry's household. The proximity to Benjamin Ishmael and the migration from Greene County of both families makes me think they had a family relationship.
2) Shadrack's will, proved in Shelby County, Tenn in 1841 includes a clause: Item ninth. It is my wish that my executors pay to the heirs of Benjamin Ishmail two hundred & twenty three dollars when they call for it.
3) The names Shadrack, Henry, and Micajah, which are this Cross family's traditional men's names, also appear in the descendants of Benjamin Ishmael, and I've even seen a Dorcas among those descendents. Actually Shadrack's name seems to come from his mother's Morris family.
So, FWIW, I'd like to offer these tidbits to the Ishmael researchers. I also have a couple of questions based on my guess/belief that Benjamin Essman and Benjamin Ishmael are the same: is Benjamin's frequently published birth date of 1790 documented? This would make him 15 at the time of the marriage, and his father Thomas would have been over 50 when he was born. Could Benjamin have been older, or could there have been another generation between Thomas and Benjamin? Also, there seems to be a large gap in the ages of Benjamin's children, so even if the Greene County marriage record is his, its possible Darkis may have died after a couple of children and Benjamin remarried in Giles County.