it seems you can call tensor_handle.Resolve().Data() to get a void *.  From the AbstractTensorInterfaceyou could also usedown_cast`? There are examples such as
Status TF_TensorToTensor(const TF_Tensor* src, Tensor* dst) {
  return tensorflow::down_cast<const tensorflow::TensorInterface*>(src->tensor)
      ->ToTensor(dst);
}

which might help.