LM317 - bringing down to 0 volts (with a uC)
My aproach is probably too much in the way of parts for you, but here it goes:  Use an opamp ( dual supply ) to generated voltages with an offset.  Feed this to the adjust pin.  You will use very little negative current, perhaps you can get it from a charge pump running off a square wave produced by the microcontroller, else a 555 timer might be used.