TI LM3488 SEPIC, adjustable power supply down to 0 volts
The FB pin regulates to about 1.26 volts so this would be the lowest voltage normally attainable at the output but you could experiment with a bias resistor fed from an independent stable reference of (say) 2.5 volts and injecting current into the RF1 and RF2 node.

This will con the chip into thinking it is in regulation and the output voltage will be naturally lower. This may or may not work of course. Doing the same sort of trick with the ISEN pin may also prove useful in being able to alter the current limit.