In my neighborhood, there's a guy who stations his rig in Wadabri Koen on a little bridge crossing the Zenpukiji River. He sleeps in the cab getting out on rare occasions to sell a spud or two or to stoke the fire that's burning on the bed of the pickup. Some vendors fire up the tubers with broken furniture (which I suppose is at a premium as folks have already burned them up for New Year at the temples). Our vendor seems to use the detritus from construction sites - old scraps of treated lumber lending a particularly complex and modern taste to the roasted skins.
The basic theory here are makeshift stoves and a central flue, so the heat remains steady, with a side chamber lined with little stones. The imo are placed upon on the stones. Indirect heat and long cooking make for particularly sweet and densely textured sweet potato. They're a bit overpriced. 200 for a small one and up to 600 for a custom supreme, but on a particularly biting day, there's nothing much better than sinking your teeth into a such esculent hypogenic vegetable matter. Well worth it!
The satsuma imo (薩摩芋 - the Japanese sweet potato) are a dense yellow-fleshed potato with rose-colored skin that turns an appealing burgundy color when well roasted. They are somewhat drier than your usual New World sweet potato. Seems they made a circuitous journey to Japan. They went from the New World to China and then landed in Okinawa sometime in the early 17th century and then worked themselves up to northern islands. They are also called kansho (甘薯), ryukyu-imo (琉薯 - after the place they first found themselves in Okinawa), and karaimo (唐薯芋 - Chinese potato).