According to this site , the old Xcode 3 syntax highlighting plugins were based on an unofficial and unsupported API. Xcode 4 is a complete overhaul of Xcode, so the API is unlikely to exist already, and as far as I know, no one has yet discovered any such API, so it looks like you might be out of luck.
The good news is that Xcode plays well with external editors (of which there are quite a lot of Lua), and any changes you make in the external editor are instantly reflected in Xcode.