The format support is written specifically so that it is extensible.  You can write your own string subclass that extends the formatting mini-language with whatever features you find useful.  There are too many variations on what might be useful with regards to case transformations for this to be a sensible addition to the language itself.  Much better that an application create use-case tailored facilities.

(Note, by the way, that new features can only go in to 3.3 at this point.)