Is handoff / continuity (iOS 8+) supported in iTunes to seamlessly switch playback between Mac and iOS? I'd love to be able to control my currently playing iPhone song from my Mac. Does anyone know if handoff / continuity is supported in iOS 8 or 9 Music.app?
Handoff is not supported in Music.app, however your use case (control media playback on your iPhone from a Mac) would be more along the lines of what the Apple Remote app does, just in reverse.

Tunes Remote allows you to control one device from another device or a Mac. I have not used the software, so YMMV.