Even if you disable or change the Mavericks keyboard shortcut for Special Characters…, OmniFocus 2 will still report that the shortcut is reserved by the system when attempting to assign ⌘+⌃+SPACE to Quick Entry.
You can force this keyboard shortcut by entering the following in the terminal:
defaults write com.omnigroup.OmniFocus2 QuickEntryHotKey -dict key -int 49 flags "command|control"