You got me to thinking about how to make a stove more directional, for a solo hiker/camper. It is true what you say, but only if the outside surfaces are the same temperature. The top is usually even hotter, which reinforces your point.

So what if the stove was better insulated all around, even on top when you don't have a pot on it, and then you opened the door to allow heat to radiate out directly in that drection only. With more insulation on the other sides you would still maintain a high combustion temperature. If the stovepipe was small enough in diameter and not to tall in height you would not get too much airflow, and your stovepipe would be lighter in weight. You would probably still want a baffle.

So how much heat can you radiate from a 4"x4" opening?

Heat Tranfer Rate, q = 5.6703 10-8 (W/m2K4) x A x (Ts^4 - Ta^4)
(Area, A in m2; Temperature, T in degK)

Area, A = 4"x4" = ~ 0.01 m^2
Ta = Ambient Temperature of say 300K
(80F surface temperature of person sitting in front)

Ts = Stove Temperature of 260F to 800F depending.
400K = 260F: 10 watts = 9 kcal/hr
500K = 440F: 31 watts = 27 kcal/hr
600K = 620F: 69 watts = 59 kcal/hr
700K = 800F: 132 watts = 113 kcal/hr
800K = 980F: 228 watts = 196 kcal/hr

For every ounce of wood burned (roughly 1"x1"x4") at 20% moisture you will get about 400 BTU = ~ 100 kcal, so if you can achieve 10% efficiency in terms of direct radiant heat, if that is your aim, it would require 10 ounces of wood per hour to transfer 100 kcal/hr, or 30 ounces per hour to transfer 300 kcal/hr. Of course there will be other useful heat by thermal radiation and convection to the space and other surfaces, such as you sleeping bag, hanging clothes, cook pots, fuel supply, that sort of thing. It shouldn't take much heat for a small space, but with a wood stove you can't really run it all night like a candle or oil lamp, so its nice to be a little larger to get a really good drying out heat in a short span of time. Still, doesn't take much of a stove to do that for a small space. It would be interesting to go even smaller, also with a smaller diameter stovepipe. Say a 4"x4"x8" firebox or even smaller. Perhaps a vertical batch load, so maybe 4"x4"x4" of fuel, and 4"x4"x4" of space above that, but designed and packed in such a way to burn down uniformly and slowly.