Hi and welcome to devnet,
You should rather ask this on the interest mailing list you'll find Qt's developers/maintainers there (this forum is more user oriented)
From my point of view, Qt supporting old Windows version up to XP, it's rather unlikely that a WASAPI backend will be implemented soon (AFAIK it's available starting Vista). Also, Qt is not a low latency audio framework, it happens to work on OS X because the OS is already built like that.
HOWEVER I don't say I can't/won't be done, if a backend using WASAPI is contributed, it might very well be integrated.