Produced a branch doing this. Not yet merged into master.
disabled powerdevil service to prevent occasionally message "no valid power management backend plugins are available" - https://phabricator.whonix.org/T297:
But that alone is an incomplete solution. The battery monitor still appears in systray by default. And worse, when you click at it, there will be an error message, because power devil has been disabled. I am onto something to get rid of the battery monitor as well. Let's see.