Diospyros is a very big genus, and the species occur in Australia, PNG, South Pacific, SE Asia, India, Africa, Americas off the top of my head.
There is a possibility that two species are closely related enough to be graft compatable, as well as the chance that they are not.
Some of the Australian Diospyros seem similar to D.kaki, and some quite different looking plants.
Kaki can graft to D.americana and D.lotus so cross species grafting is a possibility.
From what I have seen D.texana seems fairly different to D.nigra, but that may not mean they are not in the same sub group ?
Maybe it is worth the experiment ?
I have not heard of D.texana being used as a rootstock for D.kaki, which would seem a useful combo in some areas ?
I believe there are also other Diospryos from further south than Texas.