I eventually managed to spin up all the processes (libvirtd, virtlogd, virtlockd) individually and under root, which also helped exposing the read-only socket. I gathered from reading through the libvirt source code that the read-only socket is only activated under privileged mode which has led me down that path.
With all this running, I went back to my automation efforts with Vagrant libvirt and realized that it wants to make a call that doesn't seem supported on macOS (yet):
Call to virDomainCreateWithFlags failed: Unable to create TAP devices on this platform: Function not implemented
^ which I suppose confirms the mentioned worries of Michal
So I can say that while running libvirt on macOS isn't super smooth it's possible. :)
The sharp edges around permissions and launchd plist files is something that can be smoothed out eventually and that is probably something to resolve on the distribution side (e.g. Homebrew) anyway, not necessarily on libvirt's.