Thin hub-centric spacers are not easy to find, and will not be available for all applications. Using a 2mm thick spacer, I would think that normal hub-centric rings would still engage enough onto the hub centre.
@chrisp , all this talk about how hub-centic rings work has raised another question for me to ask. Is the centre hole in the 2mm spacers large enough to clear the angled part at the base of the hub centre. It would need to just clear it to be able to sit perfectly flat against the wheel mounting surface. It looks in your pics like it might, but it doesn't hurt to ask. It could be your problem if it is not quite clearing.
Well, I'm stumped. Please let us know if you find the solution. I am very interested as to what is happening.