Probably take a little longer but full upgrade to ceres on a base ascii installation from netinstall is the way i would do it.

I would do the same.

Simply install Devuan with nothing, only the base system, and then edit the sources list to ceres and do a dist-upgrade.
Later add your window manager or preferred desktop.

I attempted to follow the steps I have used previously with the Debian mini.iso, but there doesn't appear to be an option on any of the Devuan isos for an Unstable installation.

Devuan has a mini.iso as well (it's in the package mirrors, not the iso mirrors).
The installer allows you to select testing and unstable as well as stable, but installing unstable (ceres) doesn't work.

The installation fails with a conflicting packages error when it tries to install both eudev and udev.

Installing testing (beowulf) from the mini.iso mostly works, but there were some hiccups there as well.

That appears to have installed something that is mostly buster with devuan sysvinit and keyring...

I'm not sure the mini.iso is an officially supported devuan installation medium, though ;-)

Could one drop to a shell in the netinstall iso installer for ascii and modify the /etc/apt/sources.list ?

I have my doubts. I don't know the exact inner workings of the installer but i am quite sure it installs the base system using debootstrap (or rather specialized version of it) which by default doesn't care about sources.list. The installer might be smart enough to get it's settings from there but again i doubt it. Also i am not sure sources.list even exists before /target is created (likely when debootstrap is run).

Probably take a little longer but full upgrade to ceres on a base ascii installation from netinstall is the way i would do it.

Hello,

The question is essentially just what the title is asking: is there any way to directly install Ceres/Unstable?

No.  There are no ceres isos.  AFAIK, you'll have to do it via an upgrade.  Maybe someone else will have another idea for you.

The question is essentially just what the title is asking: is there any way to directly install Ceres/Unstable?

I understand that is unconventional and usually not recommended, but I do know that it was possible with the Debian mini.iso (AKA the netinstall image) to install Sid/Unstable directly via the expert installation options.
I attempted to follow the steps I have used previously with the Debian mini.iso, but there doesn't appear to be an option on any of the Devuan isos for an Unstable installation.

Thanks in advance for any help!

Here is a link on how it is done in Debian
How to direct-install Debian Sid
Here is a link to the Debian mini.iso
mini.iso
Here is the official Debian wiki talking about installing Unstable directly
How do I install Unstable?