Ok so first off besides double-click the hardware button to enable theatre mode I’ve seen at least one implementation of this on the ZenWatch 2. The ZenFit sleep tracking app automatically enables Theatre Mode upon launching.
Further, I’ve found this blog post talking about how to set the global variable for theatre mode.
Tested on LG Urbane watch. No such luck; I suspect since my watch does not have a su binary, (the ADB hack requires it,) this will do nothing until I've rooted my watch. I will attempt this and post back. Thank you for the attempt.