Re: Possible to shut down an s6 service via command rather than signal?

Brett, I had a similar issue.  (Please note: I do not use qemu and only 
use FreeBSD/HardenedBSD, with lots of lightweight jails).

I'm surprised you need to write a catcher for signals as that should be 
caught by your init (Id:1) process which should be graceful?

I replaced init with s6-svscan, so here's what I've done, for 
shutdown.sh (and lots of other maintenance things)
s6-rc -v 9 -da -t 10000 change > /dev/console
s6-rc -v 9 -Da -t 20000 change > /dev/console
sync 2>&1 > /dev/console

For .s6-svscan i have scripts to capture signals for:
SIGINT SIGTERM SIGUSR1 SIGUSR2 finish crash
Each does similar things except finish, which perform
sync
umounts
lots of other things (graceful jail shutdown, transmit closed log files) 
and initiates a halt or reboot depending on the signal.

An example of signal capture: .s6-svscan/SIGUSR2
#!/usr/local/bin/execlineb -S0
fdmove -c 2 1
redirfd -wnb 1 /dev/console
foreground { echo Executing SIGUSR2 }
exec /s/scan/.s6-svscan/finish USR2

You should be able to do everything in the context of s6 :)
Good luck.