The graphical user interface components of macOS are proprietary software, as are a bunch of other libraries included in macOS that software packaged for macOS is likely to use.

So if this was a goal of PureDarwin then they’d have a lot of work ahead of them in order to implement those APIs.

This wiki page on the repository seems to indicate that they are not intending to do that:

https://github.com/PureDarwin/PureDarwin/wiki/Graphics

> While Darwin is the operating system underlying Mac OS X, the graphical environments of PureDarwin and Mac OS X are very different. This is because most graphical aspects of Mac OS X (especially Quartz and Aqua) are proprietary, closed-source software and are not available for PureDarwin. Hence, PureDarwin uses a more traditional UNIX-like graphics system (X server), waiting a better alternative.

https://en.m.wikipedia.org/wiki/GNUstep may help with some of this, but it is hard to keep up with a moving target.

I would be interested to know if it might be possible to eventually install XCode Command Line Tools or similar, and then use it to sign DMG packages?

At the moment having to keep a Mac solely for the purposes of signing software packages is very frustrating.

clear! depends on which