Right, thanks for the advice everyone- I haven't checked with the tire off yet, but I will before I take it out for another ride. As for the braking problem:
OP:
Is the sensation somewhat like a split-second snap - like passing over the seam, or is it a more lengthy modulation, like a whole section of the rim is rubbing through the pads?
Just for a split second- as I said, I've trued the wheel as best I can and there's a very clear point at which the brakes catch.
Does this happen if you hand crank the bike in a stand, then brake the wheel? Or, just when you're riding it?
This was actually how I figured it out. When the bike is in a stand and the front wheel is spinning, it's possible to squeeze the brake lever only slightly until the wheel begins to catch at this one point. There's an area about a thumb's width directly at the seam, or at least where I assume the seam is (it's impossible to see, but it's directly across from the valve stem). While this area is between the brake pads, it sticks. When this area slides out of the pads, it spins freely. Again, this "trouble patch" is only about a thumb's width.
Check for a crack with a black permanent marker. Paint up the area, then wipe it down with Goof-Off. Don't soak it down or you will rinse out the ink from any crack. If there is a crack, some of the ink will remain in it and will be easier to see.
I'll try that, thanks!
To try to respond to everyone else: The seam isn't out of alignment, or so it seems (sorry). The wheel is true. The brakes have been adjusted wide enough to eliminate brake rub and the wheel spins fine, there's just an annoying modulation in the braking just around the rim seam and I was concerned about damage to the headset or (gasp) ovalizing the headtube. I'll remove the tire and check for cracks on the inside of the rim, ink the seam, etc. etc. but failing that I'm willing to believe that the wheels (while old) are low mileage and that the bulge at the seam hasn't been worn down yet.