Would it be possible to add support for devuan into Gentoo deboostrap packages? Besides the distribution codenames, it would likely require incorporating some other code from https://git.devuan.org/devuan-packages/debootstrap including but not limited to making sure systemd is not installed when not asked for (there is related debian bug on https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=668001 but I didn't found where the discussion moved from then, if anywhere).
Or did they diverge to the point it would be better to make them two packages?
 suggests that you use debootstrap to install some Debian release (in a chroot obviously) and then tweak it to arrive at something resembling a Devuan release. Support for Devuan should be merged at debootstrap upstream, and not in Gentoo.