Yeah, not all numbers will be following the rules either. That is more or less the story of an internationalisation engineer, you find all the places where people were assuming assumptions and wind up ironing out a lot of bugs along the way. The full story is that I'd be looking at doing this in base 12, so I am going to be using the additional code points U+218A and U+218B for 10 and 11, and have a growing collection of fonts with these code points included.
Supposing I wanted to change the way all numbers are displayed, is there any feasible way to go about this at present? Not talking about a font, but say if I wanted to display everything in a different number base, is there a sensible way to go about it? For example, in a standard Java application, usually it would be governed by the NumberFormat, which I could conceivably replace by providing a LocaleProvider. Does Minecraft happen to behave as a good citizen in this regard?