After discussing with SG16, we figured out that formatting into char16_t isn't likely to happen even in C++26 (although the situation still may change, as we're only mid 2024 now).

So, if we want to provide an std::format-like API for formatting into QString, we need to implement it on our own.
In the SG16 mailing list Thiago suggested that we provide our own qFormat(), which would act like a wrapper on top of std::format_to, and also a custom OutputIterator, which would allow us to pre-allocate the needed space in the buffer, and also format directly into the buffer.

That, of course, means that we also need to provide support for the custom output iterator for build-in types at least.