After a little further investigation I can confirm my suspiscion: using a LABEL specification rather than the standard md0 node allows the boot process to complete successfully even if the sleep is removed from the udev script.
-- initramfs scripts kill initial "coldplug" udev processing before devices needed for root filesystem are loaded https://launchpad.net/bugs/89366 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
