I masked all versions of udev above 171 because of the
/usr merge. There are some nice things that udev does, mainly ensure consistent matching of device names to actual hardware. However, (for any of my systems anyway) the plug-and-play stuff isn't really needed until after the OS is up and running. I really don't need the system to do anything with random USB thumb drives or game controllers at boot time. The thought had occurred to me to simply go back to a static
/dev.
I've looked into alternatives. There is tmpfs-based
/udev support in the kernel that seems to work well (minus security settings). mdev, from busybox, seems to work well. I've also looked at
mudev, though not much past the project page.
If Gentoo devs are forking udev into a new project that supports separate disk partitions for root directories, then I'll fully support and install it.