Okay, I seem to have achieved this by adding:
To the adh-user-add recipe that everything is depending on (including the image recipe).
I'm a bit confused as to why that was such a hassle to achieve something I would have thought was a simple adding of a few users. Definitely would've appreciated a warning that the build system was attempting to add an empty package or something like that. I wouldn't think I'm on the fringe's of Yocto development here, but perhaps my use case is resulting in an 'empty package' being created via my image recipe that most other use-cases aren't triggering?
I would be very glad to hear any input despite having this resolved.