Is that an artifact of the way the plugin handles the command, or an actual restriction on the hardware?
Unfortunately that seems to be the hardware; it basically ignores all commands sent without any response indicating it will not process (other than the status not changing for the selected command). I've been toying with having the plugin automatically send a Zone Power On command if you submit another command to a powered-off zone. Thoughts on that?
This is part of the reason that I don't usually turn zones on/off but rather just mute them.
I forked your repository on GitHub, made the changes in that fork, and did a pull request. Trying to save you a little work.

Awesome, thanks... I was literally about to open that to take a look at fixing it and came here to read the forum first. Got it merged in!