RE: Representing play button available state by blinking -- maybe a better way for skins is to gray out the button or otherwise display a pixmap that indicates it is disabled. Mixxx already has this as an undocumented feature for WKnob and WDisplay (The <OnOff> connection type). Other widgets should get support too:
https://bugs.launchpad.net/mixxx/+bug/1180872

I realize this is also useful for MIDI and it models the hardware CDJ the user asked us to emulate so maybe both should happen.