Proceed as follows (assuming you don't use a separate /boot partition and everyhting is on /dev/mmcblk0p1):

apt-get -t ascii-backports install linux-image-armmp-lpae
cd /boot
mv boot.scr boot.scr.ORIG
mv boot.cmd boot.cmd.ORIG

Now adapt your boot.cmd.ORIG and adapt the paths/add a line for the initrd. In my case:

setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p1 rootwait panic=10 ${extra}
load mmc ${devnum}:${partition} ${kernel_addr_r} vmlinuz
load mmc ${devnum}:${partition} ${fdt_addr_r} boot/dtbs/${fdtfile}
load mmc ${devnum}:${partition} ${ramdisk_addr_r} initrd.img
bootz ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr_r}

Then install u-boot-tools:

apt install u-boot-tools

And create an updated boot.scr:

cd /boot
mkimage -C none -A arm -T script -d boot.cmd boot.scr

On the next reboot, the kernel you previously installed should be booted (the hardcoded kernel does not hurt as it resides in a different path, but you can wipe it away if it bothers you). Verify you are running the backport-kernel with uname -a:

root@devuan:~# uname -a
Linux devuan 4.18.0-0.bpo.1-armmp-lpae #1 SMP Debian 4.18.6-1~bpo9+1 (2018-09-13) armv7l GNU/Linux

I wanted to run Devuan ascii on armhf as I did previously with jessie. However, it seems like the new embedded images do not use a packaged kernel anymore. Instead, the kernel is built "hardcoded" at image generation time. I noticed that the regular kernel packages are in the repository as well (linux-image-armmp-lpae) but I didn't figure out how to configure u-boot for using them. Seems like the Devuan kernel does not use initrd either, so at least I would need to load this in before booting.

Or asking more in general: What is the way Devuan users are supposed to (automatically) receive kernel updates for production server running on armhf boards?

Cheers