Has anyone done a resident C++ library before?
Nope. You'd be the first.
Keep in mind that the interface "feature" of OS 4 is not available on OS 3 so you wouldn't be able to use any object-oriented features within the library code.
I'd be interested in experiences and sample code if possible. Both for ppc and m68k.
Are people using C++ on Amiga to any extent btw? Or is it still mainly C and other languages?
Not really. It's mainly C and other languages. I rather like AmigaE but the library mode is severely broken on it.