Some of them actually do support darwin and we have a function in our company overlay:

    withDarwinEnabled = drv:
    drv.overrideAttrs (oldAttrs: {
      meta = with super.stdenv.lib.platforms; {
        platforms = darwin ++ oldAttrs.meta.platforms;
      };
    });

but really, we've only run into a couple of things we've cared about that are Linux-only, and just PR'd fixes upstream and duplicated them to our overlay temporarily.