Hello Andy,
The lowest you can go on input voltage before you hit duty cycle limitations is 2.4V. I have attached a webench schematic here. (Please visit the site to view this file)
Another part that slipped my mind is the LM2623. It is similar to the LM2621 in features, but it's duty cycle can be programmed by using a capacitor. The maximum duty cycle you can obtain is 90%. I would also suggest looking at that part. The challenge in using that part is that the capacitor that helps programming the duty cycle is selected on a trial and error method. Therefore Webench cannot help you there. You could still refer the datasheet for your requirement. I hope this helps.
Regards,
Akshay