WinRT does indeed throw the old Windows Win32 and Win64 API away and even most of the old NET API’s (there is some cohabitation however in regards to NET) There is not even MIDI support in RT 8.1 SDK, they promised it and beta versions had rudimentary MIDI support but it had been thrown away in the release. If you want to use a hardware keyboard you will have to write application specific drivers for the keyboard for instance. Which is nuts, you cannot write generic drivers because there are no generics. Real time, low latency audio support was added in 8.1, but not a full implementation , just enough to get Skype going.
There is the option of communication with old windows API’s via bridge software and that is what the MS team recommends for communications stuff, but because of the real time aspects that is not acceptable for audio programming
I had actually started writing a simple MIDI editor for my old Kawai synths before I actually realised that there was no MIDI support available …