Unclear - unlike most phones, it has no removable storage, and my impression (having only heard descriptions of the UI) is that there's no way to run arbitrary code on the device. Unless it has accessible serial ports or some netboot mode hidden in it, or in the very scary case that the software update mechanism is insecure enough to MITM it yourself, it seems like it would be difficult to get a Chromecast to run anything other than Google's software.