edit: modkey + o moves windows between displays — that pretty much solves the whole thing. Sometimes patiently reading though a man page solves your problems more efficiently than asking Google for help. (Thanks to Wolfgang for informing me about the key binding.)
Getting LibreOffice Impress' extension presenter-screen (aka "Presenter Console") to work when using awesome turned out to be a bit tricky.
So, if youre using awesome and want to use the extension, here's how it works:
Assuming you're using a laptop with a resolution of 1366x768 and present on a beamer, connected via VGA with a resolution of 1280x960. (Use xrandr to find out the actual output and resolutions.)
1. Display setup
Your own screen: xrandr --output LVDS --mode 1366x768 --primary
The beamer: xrandr --output VGA-0 --mode 1280x960 --right-of LVDS
Make sure the extension is recognized: Tools -> Extension Manager
Set the presentation display to "Display 2": Slide Show -> Slide Show Settings
Place the Imress window on the beamer display and start the presentation.
You should now see the presenter screen on top of your presentation's slides, both on the beamer display.
With your cursor, hover over the presenter screen.
Press modkey + m (maximize window) twice.
Move the presenter screen to your laptop's display.
This all would be much easier if there was a key binding for moving (maximized) windows between displays, not only tags. I'm not aware of one.