I think I found the answer:
I used the wrong terminology when searching (“unofficial” instead of “unregistered”) but after re-reading the pkg documentation I got it right.
I opted for adding a before_install script to CI like this:
julia -e ‘using Pkg; Pkg.add(PackageSpec(url=“GitHub - DrChainsaw/NaiveNASlib.jl: Relentless mutation!!”));’