I kinda got it to work through some doctoring.
I took the files from an old build and replaced the default.xbe with the one from the new build. Which worked, except the LED kept cycling through red, amber and green.
I went back and added the LED configuration lines into the old config.xml and tried again. Didn't work. It got to the Xbox logo load screen and then did nothing.
So I decided to just try things again with the cycling LED problem, deleted the LED lines from the config file, copied the RealPlayer .dlls into the dll folder and burnt the disc. It started up, as before.
I tried to run a few .rm files, they played, but with zero audio. There wasn't even a toggle bar on the audio adjuster, so I'm guessing it wasn't finding an audio stream or something, despite there being one. Other than there being no audio, the .rm files seemed less glitchy visually than in previous builds and the run-time bar was a lot smoother.
It looks like I'll have to revert back to an old build. Unless someone can tell me a solution to my initial problem?