Not only that I believe that your claim is true, it makes even sense to me.
Say, I wanted to typeset romanised chinese with numerical tone notation,
I'd probably choose old style digits in lowercase context, but modern style
ones in an all caps headline.
If I remember right, Zhuang (a minority in southern china) uses a similar
system, however the tone letters are a little _verfremdet_ digits (3 is
cyrillic z, 6 cyrrilic b, the others have a little peculiar design).