You can put a coaster on any Moulton if you like. I have a Shimano type D coaster on my “how light* can you make a road legal F-frame?” bike, because it’s lighter than running a singlespeed hub and separate rear brake. I have a torque arm bracket brazed to the LH rear fork blade but I don’t know if the Stowaway had a braze-on or just a clip.
I thought about a Duomatic 102 but they’re not ideal; they step up from direct drive, so for efficiency you have to have a cruising gear and a little-used downhill gear. You can gear them so the high gear is the cruising gear and you get a nice climbing gear, but that means you spend most of your time in the lossy gear. 3-speedz are best, really.
*9.75kg/21.5lb including pedals