Lights

The circuit you're proposing won't deliver the constant current required by non-linear devices such as LEDs.

I haven't proposed a circuit yet, I just said that I'll use a transistor :)

I was thinking of starting with this:

I think it should give reasonably constant current, so long as the transistor's current gain is fairly stable. I might replace the diode/resistor divider with a proper voltage reference buffered with a low power opamp, to save a bit of current there.