I've been coming at Zachariah through his various children and I think I *MIGHT* have found another MATTOX/MADDOX connection.
Zachariah & Linda had a daughter named Nancy ELLIS, born 1815. In the 1880 Census for Union Hall, Franklin Co, Virgina, Nancy is living with John W MATTOX & Julia (clearly not a MATTOX by birth). Nancy is listed as "Aunt" in her relation to head of household. That means that John W MATTOX or his wife Julia MUST be a grandchild of Zachariah ELLIS.
Tracking down Julia, I find a Julia Ida ELLIS (b 19 Aug 1854 in Franklin County, Virginia, USA - d 09 Jan 1937 in Franklin County, Virginia, USA). Her parents are listed as Warren ELLIS & Julia ELLIS on her marriage to John W MATTOX on 19 Oct 1871, in Franklin County, Virginia. Julia is 18, John is 23, both are single. John's parents are David P MATTOX & Julia J.
Warren Preston ELLIS married Julia KEMP 11 Dec 1841, in Franklin County, Virginia. I have yet to find Warren Preston's parents but I think they're likely to be Zachariah ELLIS & Linda DEHAVEN.
Just thought I'd share what I've found so far. I'll keep digging at my end, but I was hoping you might have some information on John MATTOX & Julia (or John's parents).