Most likely the violation of the NIFTI file format in your file header that prevents loading into Slicer is that the dimensions are incorrect. Dimensions are hardcoded in NIFTI. 4th dimension is always time: it is set to 3 in your file (correct value for a 3D displacement field is 1). 5th dimension is the dimensionality of the displacement vector: it is set to 1 in your header (correct value is 3 see
here: dim[5] must be the dimensionality of the displacment vector (e.g., 3 for spatial displacement...)
I’ve implemented this now - pull request submitted.