Hi @crocodoyle,
I would say that your first hypothesis/question is the correct one:
The commit history of the itk::Testing::ComparisonImageFilter shows no related change since the above post, so I assume this is still to be done.
Not sure if the itk::SubtractImageFilter is adapted to work on vector component images. Maybe others can elaborate on this.
Opening issues, as well as providing testing data, would help bringing attention to these aspects, and folks in the community may be able to consider the feasibility of addressing them.