I've taken a closer look at the Mac graphics, and noticed a few things:
1. Every graphic (a terrain piece, or a frame of an object / lemming animation) is a separate file. Annoying, but manageable.
2. The whitespace (or rather, blackspace) on the bottom and right of every piece - but not the top or left - is culled. This is probably the easiest issue to deal with.
3. No matter what rule is used to translate 2x2 high-res pixels to one low-res pixel, the terrain is not always physics-identical. For example, see the attached image - yellow pixels are solid on Mac but not DOS; red pixels are solid on DOS but not Mac; green pixels are solid on both; black pixels are not solid on either. (For the graphic, I translated upwards - a solid pixel in low-res = all four pixels solid in high-res.)
The 3rd one is the biggest issue, and I suspect that manual work would be needed to resolve this.
With this being said: I have not by any means confirmed that WinLemm's graphics don't have similar issues. They very likely do.
If issue #3 does exist to a significant extent in both Mac and WinLemm, I will probably not make the effort to actually prepare high-res graphics myself, as I feel my time would be better spent elsewhere. Of course, I'll still implement the support for it in NL - it'll just be up to someone else to actually convert / prepare the high-res graphics.