Fred, I must respectfully disagree with you. I agree that the same amount of heat is generated, but I disagree on how it is dissipated. I think you misunderstand Sheldon's point, below:
Sheldon Brown writes, "On long, straight mountain descents, ... pumping the brakes, alternating between one and the other, will briefly heat the surface of each rim more and dissipate more heat before it spreads inwards to the tires." (http://www.sheldonbrown.com/brakturn.html) Also true, but a tiny effect. How long does it take heat to pass through the 1/16" of aluminum between brake block and tire bead? Less than a second, I'd think.
The problem is that you're thinking in linear terms, and heat and fluid mechanics are anything but. When you ride the brakes, or lock them up, you're not going to reduce the rims' temperature -- you keep dumping heat in, so some is lost to air cooling, and the rest goes into the rubber of the tube, which eventually flows into the air in the tube. If you pump the brakes, you'll generate a heat spike, but when you let off, MORE heat is lost to the air flowing around the rims than if you kept dragging the brake, because the temperature spike is higher, heat flow is nonlinear, and you give the rims a few seconds to cool off. Also, this cooling is more efficient while you're riding than when you stop, because the air velocity around the rim is much greater while you're rolling than you're stationary, and convection is much more efficient at heat transfer than radiation (which is why the color of your rim doesn't matter).
The net result is that the air temperature inside the tubes doesn't get as high, so the pressure stays lower.
If you want to try this out, you can make a couple of runs on a short, steep hill, and compare rim temperatures between riding the brakes and alternately pumping them. Or, if you just want a demonstration, brake most of the way down to a point where you can safely let the bike roll out the rest of the way. Stop and feel how hot the rims are. Wait a minute, or three or five minutes, and feel again. Go back up part way, stop and feel the rims; then let it roll the rest of the way down, which will take 15-30 seconds, stop, and feel how cool the rim is now!