Ok I took a look at the Atkinson dither. It is interesting that it does seem to get subjectively better results on some images because algorithmically, it looks like Atkinson's main concern was performance: his dither doesn't require any multiplications. I'll include it as an option in the next release.
To anyone reading - is there a preferred file format for DHGR images? I'm leaning towards a straight memory image, like HGR, but even then, should the file contain the aux memory image followed by main memory? The other way around? Interleaved in display order? If there's no "standard" then I'll probably go with the first of the options I just mentioned.