I've tried with my gentoo /usr and (or) /var partition but udev seems to have some problems to initialize devices.
As many of you will be aware, udev upstream is moving to requiring /usr mounted before it starts. Upstream (including kernel developers) advocates an initramfs across the board for Linux distributions, but definitely in the case where /usr is on a separate partition. This is not down to udev itself, but helper scripts for devices which often live in /usr or sometimes /var, and aiui the hardware database is also on /usr (i guess that bit is down to udev itself;) [...]
If you include /var (and /usr) in / you could solve.