Oh, turns out neither of my workarounds will work. As far as I can tell livery artists are completely unable to make a livery with ANY form of registration at the moment, which is a shame.
We can't paint the livery in the texture because STARSHIP_EXTERIOR3_ALBD.PNG is mirrored. While this is good for texture efficiency, it is limiting for livery artists.
We can't add a model attachment with the static tail reg on a mesh, because the model needs to animate with the engine nacelles when the wing flexes. I don't think this is possible to do by adding model attachments to the Starship model, although someone may (hopefully!) prove me wrong.
We can't use the dynamic reg, because it simply doesn't work in MSFS 2024.
It's a bit disappointing, we can't really do any paints for the Starship - although I still absolutely love flying it!