You're correct with WDT daemon.
I have only found one work arround for conflicting systemd-tools and udev-oxnas, is not quite elegant but it does the trick. Is just pinning those packages, i added this lines to my /etc/pacman.conf:
IgnorePkg = initscripts udev-oxnas systemd-tools mkinitcpio linux-omap linux-headers-omap
(Last 2 packages are optional)