Update: Our latest effort is a complete chromecast v2 receiver, called YouMap Chromecast Receiver, works very well on Android devices, including FireTV: http://forum.xda-developers.com/hardware-hacking/chromecast/app-youmap-chromecast-receiver-android-t3161851
We just released a chromecast emulator on iOS, freely available from App Store:
https://itunes.apple.com/us/app/rplay/id692511073?mt=8
Spent a few days to emulate Chromecast inside rPlay, finally it can find the rPlay (on Raspberry Pi) as a chromecast device, and it can send youtube URL redirection. still long way to go, but seems possible.
Attached is a screenshot of Youtube on Nexus 7, the Chromecast button is shown, when clicked, it will show rPlay as the device.
More info on rPlay: http://www.vmlite.com/index.php?option=com_kunena&Itemid=158&func=view&catid=23&id=10991
We will release chromecast emulators for windows/Linux/Mac/android/iOS in September.