While udev can be built from the systemd sources, it still requires building dependencies that are NOT needed by udev.

This was silly and even though a patch was created specifically to address this, it was nak'd by the systemd developers.

Disclaimer: I've been a Gentoo developer for about 7 years now. I'm not directly working on the fork but I am testing and throwing in suggestions for using it on ARM devices where it doesn't make sense to have everything systemd provides, but the devices are more powerful and busybox's mdev is underkill.