I did a bit of investigation into this matter myself, but the solution I came up with is far from elegant. Let's start with where runit currently stands in the Debian/Devuan universe. Runit can be used as PID 1, but for booting and shutting down, it depends on three different init scripts that in turn depend on sysvinit:

Booting:

/etc/init.d/rcS - Executes the symlinks to the actual startup scripts located in /etc/rc.d/S##xxxxxx with option "start".
/etc/init.d/rmnologin - Removes the /etc/nologin file as the last step in the boot process, if DELAYLOGIN=yes.

Shutting down:

/etc/init.d/rc - Main runlevel control script.

If your end goal is to use runit along with sysvinit for booting and shutting down, this is not a problem. For simplicity, however, I wanted to use runit independent of anything else. Attempting to purge all sysvinit packages along with the initscripts package resulted in an unbootable system. To circumvent this problem, I manually compiled Void Linux's runit configs along with their executables for rebooting, shutting down, etc. This allowed the system to boot again. In my opinion, Void's implementation for runit is far superior to that of Devuan's. If Void's runit could be used for Devuan, there is one other package I would like to be able to remove: init-system-helpers. init-system-helpers, from the information I could find, is only useful for switching between sysvinit and systemd. I don't see how it's necessary if runit is being used as PID 1.