Re: motion sensors: the model I've seen on sale in America is exactly that, combined with an LDR, powered by 3xAA batteries, and fairly slimline. Since I use NiMH AA's extensively and always have some lying around, I've no problem recharging these once a month or so... The light is actually designed as a wirefree installation for dark/deep wardrobes, but then our bathroom isn't much larger than that :-)
As for your requirements Nick, I suspect this thing would do just fine... however, for a public outside toilet it might have to be sturdy rather than 'neat', so I'd suggest one of the 'alarm' style sensors like Malcolm mentions, rather than this wardrobe one.
You could probably simply wire a commercial solar battery charger or two in parallel with the cells, since the solar cells would only activate in daylight, and the lights' LDR would make sure it stays off during that same time, so you wouldn't find the solar cells trying to power the lamp directly.
Damian