Android: With Tasker, you can create a custom Google Now command for just about anything. Android Wear allows you to trigger regular Google Now commands from your wrist, and now Auto Wear allows you to do the same with those custom voice commands.
The plugin allows you to trigger AutoVoice commands from your watch, among many other things. AutoWear can also be used to create floating icons, trigger actions by shaking your watch, or display custom notifications. The plugin comes with a free trial for seven days. Afterwards, the plugin costs $1.70.