sys-apps/systemd-200 now includes udev, but seems missing the /bin/udevadm -> /usr/bin/udevadm link, which causes plymouth failed as it calls /bin/udevadm
We're having /sbin/udevadm compatibility symlink already. /bin/udevadm is too much for me. I see this thing is patching systemd path anyway. Although I'd really prefer if upstream learned how to use autoconf to find tools if it really needs the absolute path.
After discussing this with Samuli, I've moved udevadm to /bin/udevadm in systemd-200-r1. This should fix plymouth for now. I've also sent a patch upstream which should make plymouth properly autodetect udevadm & systemd-tty-ask-password-agent locations, therefore make patching the latter unnecessary.
I can confirm the systemd-200-r1 fixed this problem. Thanks guys.