This brings us roughly to the present.
We're at the "it's a fucking mess" stage.
The worst part was that I had all the wiring neatly arranged and tucked away. But then I discovered that the intake manifold occupies the exact part of the engine bay that E30s normally use to pull the engine wire harness up from the block. Which meant that all the work I'd done to neatly arrange things had to be undone. Sigh.
At least I have the wire length available. The wiring in the engine bay will be tidy enough once I've installed the wire harness carrier and cover. I'll need to make a couple of brackets for fuses and relays in the glove compartment. The S54 brings four wire harnesses to the DME, so there's a lot more to put away. It won't look anything like it currently does when I'm done.