Is it possible to screencast from an iOS device to a Chromecast? Is it possible to screencast from an iOS device to a Chromecast? 

We'd like to be able to screencast in the same way you can from an iOS device to an Apple TV.
Casting (mirroring) the entire screen of your iOS device to Chromecast is not possible. Apple TV uses an Apple proprietary technology called AirPlay for screen mirroring.

Casting is only available for compatible apps which have added support for Chromecast. In compatible apps, you will be shown a rectangular button with little wireless icon in the bottom left corner, as shown in the image below: