If Debian stable is something you could compromise on, there are prebuilt images for the pocket reform: https://reform.debian.net/
Otherwise, as far as I understood, you will need to patch the kernel of your target distro yourself and provide the device tree file. See this answer and the whole thread in general: Other Linux distributions that Debian? Ubuntu?
However, since Devuan is Debian-based, you should have a good chance to get it working without too much hassle…
Note, that the debian-installer netboot images on reform.debian.net are barely tested. Recenty (December 11) @Kooda reported that the debian-installer image worked for them but since the Reform is my main machine, I’m not testing them myself, unfortunately.
Since Devuan is a Debian derivative, it should be relatively simple to adjust the scripts to support Devuan. Does Devuan patch the Debian kernel? How much do the package set around the kernel differ in Devuan? Maybe you can re-use the existing Debian kernel packages with Devuan or you can rebuild the patched source package on Devuan?
As for the mkimage.sh script, did you try running it after changing the mirror locations to point to Devuan? Did you run into any issues?