I'm kind of stuck with Obj-C on iOS but I can totally believe you could build an abstraction layer like the one you describe over vanilla C++ that would give you most of the benefits of Obj-C. Clang's Obj-C++ support makes it pretty painless to mix Obj-C and C++ as long as you clearly delineate responsibilities.

So far I haven't had to do any serious string handling in my C++ engines (mostly audio apps). The new unicode stuff in C++11 ought to make this manageable but I haven't actually tried it yet.