I think it's way too early for this sort of speculation.
I don't know where you're getting the idea that C++ is the primary way of accessing WinRT APIs. It's apparently a way of doing so, but that doesn't mean it'll be the way most developers choose.
We also don't know whether the distribution model for WinRT apps even makes it possible for us to obtain many such apps. Presumably, we won't have a way to just download the files from the Win8 store. Sideloading will most likely (but might not) be possible, but if so Wine's usefulness will be limited to apps that can be obtained in that way.
I've also been told that "not supported" doesn't necessarily mean "won't run", though even if Microsoft isn't signing all their win32 arm binaries it's likely that the arm desktop does not provide a fully-featured win32 api.
We just have to see what win8-on-arm looks like when it comes out.