Hi Toby Robertson,
Unless I misunderstood the desired goal, I don't think it is possible to customize individual members of dimensions in the Label mark. The custom options will affect all marks (see screenshot below).
You can manually move individual labels in the view, though. See the following KB article for more info: Moving Overlapping Labels | Tableau Software.
I found the following ideas that could be up-voted if they match what you want, if not I would suggest creating a new one.
I hope someone else in the Community would have a better resolution/workaround.