I have found an additional workaround in the default Ubuntu environment.
Go to System | Preferences | Appearance
Then select the Visual Effects tab, and choose None.
After doing this open System | Preferences | Keyboard Shortcuts
The Switch to Desktop N is now present for all of your desktops (at least on my system), set the shortcuts, then put your visual effects back to the original setting.
I have found an additional workaround in the default Ubuntu environment.
Go to System | Preferences | Appearance
Then select the Visual Effects tab, and choose None.
After doing this open System | Preferences | Keyboard Shortcuts
The Switch to Desktop N is now present for all of your desktops (at least on my system), set the shortcuts, then put your visual effects back to the original setting.