Xenial shutdown/restart issue with LUKS

[Mason Loring Bliss]

Hi all. I haven't submitted a bug report yet, but I've got a couple systems
using root-on-LUKS (and in one case root-on-LUKS-on-MDRAID) and systems
configured so all hit an annoying 90-second timeout on reboot where systemd
tries to detach LUKS and spin down the RAIDs and such, but can't. I've been
pointed at this:

    https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface/

...and told that I want to deal with this from a shutdown-initrd, and that
seems sensible. What I'm unsure of is whether or not Ubuntu has some
relatively straightforward way to set something like this up. The FreeDesktop
page noted mentions dracut and ArchLinux in passing. When I look at
/usr/share/initramfs-tools on Xenial I don't see anything that seems to
address this notion of doing things at shutdown.

As noted, I haven't filed a bug yet, but I can do so if there's not already
information out there on how to deal with this gracefully.

Thank you.

-- 
Mason Loring Bliss             mason at blisses.org            Ewige Blumenkraft!
(if awake 'sleep (aref #(sleep dream) (random 2))) -- Hamlet, Act III, Scene I