How can I lower the TouchBar brightness level? On the 2016 MacBook Pro, the touchbar has very high luminosity. At day it is very nice, but at night it blinds me.

It's especially vexing because I run f.lux's Darkroom mode to reduce the glare that might wake my wife up, and the main source of light is the Touchbar!

Is there any way to reduce the brightness, if not via a menu, maybe through a Plist somewhere or using the CLI?
There is no way to do this currently. However, I suggest you do as I have and submit feedback to Apple requesting this option.

Hopefully it will be introduced in an upcoming macOS Sierra update.