Steve Scott wrote:

Efficiency isn't the goal here, comfort is.

My calcs showed very little difference in energy or comfort for radiators
near or far from walls. We might redo them for windows. But it looks like
we can heat a house with 40% less fin-tube if it's below a hot air column.
That could be a nice capital savings. My kitchen has a 2'-high commercial
"convector," an empty box with fin tube near the bottom which moves more
heat than the same length of baseboard fin tube. Lots of HVAC people can
look at the specs and see that convectors have more output and pick one
when there isn't enough wall space for baseboard, but fewer seem to know
why convectors have more output, or how to build-in vs buy one.

Most humans radiate heat faster to cool surfaces than warmer surfaces.

Humans radiate at the same rate, but warmer surfaces radiate more back.

If we're using BB or radiators we'd like to have them under windows so
there's a blanket of warm air between the cool surfaces and the human
emitters so we don't feel chilly and crank the t-stat higher in a vain
effort to feel warm.

Air is transparent to radiation. Warm air makes no difference to radiation,
but warmer windows help. So do drapes and clothing. If you sit near a large
cold window, the surface cools you a lot more than if you sit farther away.

The mean radiant temp is the average of the surroundings weighted by solid
angles to a body. An 8' cube with 5 70 F walls and an 8'x8' R2 window with
an R2/3 inner air film and an inner glazing at 30+2(70-30)/(2+2/3) = 60 F
makes the mean radiant temp in the room center about (5x70+60)/6 = 68.33,
and we need to turn the thermostat up to 71.28 to make it feel like a 70 F
room with no window, according to the ASHRAE 55-2004 comfort standard, and
this increases the room heat loss by about 1.28x8'x8'/2.67 = 31 Btu/h.

If a radiator under the window makes the glazing 70 F and destroys the inner
air film, the 70 F room heat loss increases by 10x8'x8'/2 = 320 Btu/h, 10X
more than letting the window stay cooler and raising the room air temp.

While I believe the optimal placement of the radiator would be on the
floor, the sizing of the radiator is MUCH more important, and the saving
of floor space in this case is more important that the minimal increase
in heating efficiency.

And you're right on this point, Nick...

You misquoted. The OP was right, and wrong, for "placement" in closets.

Nick