The bit I was pleased with was working out a circuit that lets you use the same pair of cables for the LED power and for the switch signal from the top box. By the time I decided I wanted both switches, I'd already soldered the LEDs (plus resistors) into a two core piece of mains flex, so getting the switch signal back down the cable to the main box took a bit of thought. In the end I found a way of shorting the power very briefly through a capacitor, and detecting the current pulse using a comparator. The signal from this goes to a toggle flipflop, which switches the power on and off.
The dimmer circuit is comparatively simple - basically just an adjustable current source using an opamp and transistor.
Hope you like the pictures. I may get round to putting up a full instructable if there's interest.
Update: I've just added a couple of photos of the original hand-drawn circuit diagram, and of the notes I wrote to work things out.