I'd really like my nighttime program to be able to stop Chromecast playback (specifically, stop the music playing on a particular Chromecast Audio).

I've read about using Polyglot but this seems like adding a lot of complexity to my setup to get one feature.

Is there no other way to accomplish this?  I'm about to just put the Chromeast on a power switch and have my isy simply turn it off (and turn it back on in the morning).

Polyglot isn't complex at all except for setting it up on a raspberry pi.

Using Polyglot exposes your devices as if they were native devices. @simplextech mentioned developing an image that could automatically do this if you reach out to him. I'm sure he needs testers. 

A plug-in module works for simplicity. However a little extra work gives so many new possibilities.

I have put together a raspberry pi image that includes Polyglot-v2 pre-installed.  This resolves a lot of the difficulty in setting up polyglot.  Message me if you would like to test it as it's not yet been blessed by @einstein.42 or @Michel Kohanim for "official" distribution.