Sorry for the bit of thread necromancy here, but I'm trying to accomplish the same thing.
The goal is to use a smart TV's youtube app to serve the content and output via ARC. This is accomplishable by the Sound Town thingy, but it won't then reencrypt the signal to HDMI for output; in order to send the output back to my AVR, it gets downgraded to optical.
Now, I know HDMI passthrough is technically achievable, even when muxing content together (the xbox 360 did this for ages) so I'd like to find a karaoke mixer device that reencrypts the output signal in HDMI, so that I can avoid having to mess with cables every time I want to turn the mic mixer on or off.
I would love to know if such a thing exists, and I have a pretty generous budget if somebody can think of something. Heck, I might build it if it doesn't exist.