Don't know if there is an "official" plan to sort this. It is an
annoyance that os-prober (normally run at update-grub) detects my
(bootstrap-installed) Devuan as "Unknown Linux Distribution" then that's
what you get in grub menu.

This is fixed here by adding after line 23 of
/usr/lib/os-probes/mounted/90linux-distro :

    elif [ -e "$dir/etc/devuan_version" ]; then
        short="Devuan"
        long="$(printf "Devuan GNU/Linux (%s)\n" "$(cat 
"$dir/etc/devuan_version")")"

In whatever OS is your "master grub". If that is your Devuan system, or
you chainload it, you need also lsb-release installed OR edit
/etc/default/grub else main system shows as "Debian GNU/Linux".

Adapted from https://lists.debian.org/debian-boot/2015/08/msg00004.html

Afterwards:

# update-grub
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.16.0-4-amd64
Found initrd image: /boot/initrd.img-3.16.0-4-amd64
Found Windows NT/2000/XP (loader) on /dev/sda1
Found Devuan GNU/Linux (ascii/ceres) on /dev/sda7
Found Debian GNU/Linux (8.0) on /dev/sda8
Found Devuan GNU/Linux (jessie) on /dev/sdb1
done
#

David