Terminator does not support hotkeys to change background opacity or color:
To my mind, rewriting the config file on the fly is the wrong way to achieve the things you want. A much better way would be the dbus server that we've started to introduce. It is designed to allow things running inside a terminal to communicate with Terminator and send commands that will affect the terminal. For now it only exposes commands for splitting, but over time I hope we will be able to extend this to include things like colours and transparencies, etc.
https://answers.launchpad.net/terminator/+question/173257
CompizConfig allows you to change any window's opacity with a keyboard shortcut. The effect is different than Terminator background opacity, but it's good enough for me -- I want to glimpse the web page behind my terminal.
sudo apt-get install compizconfig-settings-manager
Opacity, Brightness and Saturation">
Related:
Terminator does support other hotkeys and a configuration file: