I've not quite figured it out, but have been off and on the idea of it. Most of the packages seem to have a GNU coreutils dependancy, which makes sense due to the wide array of things that are built on it. How much work would it take to get the option to do alternate coreutils to work so that you can skip the GNU stuff?
https://wiki.archlinux.org/index.php/Core_utilities#Alternatives
Archlinux contains busybox in repo, you can try it.
You have to ensure your new package provides all the utilities needed and then build it setting provide=('coreutils') in PKGBUILD