This should solve your problem Eelco,
under the Window menu go to Settings/Preferences and choose Hotkey Editor, a new window will open. On the left hand side of the new window choose Display from the Category list and then scroll down the Commands list to ToggleBackfaceGeometry and select it.
In the Assign New Hotkey box choose your key and combination of alt or ctrl then assign it. You may have to query your choice a few times before you get something that isn't already assigned, I chose ctrl+p for this and it worked just fine.
Voila you can now toggle the backface of any selected geometry.
Take it easy,