Well, it turns out JusticePete and I were both
The wheel/tire package was in balance (verified again on two different Hunter road force machines), but apparently the hub/rotor assembly was not. I finally solved the problem by rebalancing the wheels while they were mounted on the car to compensate for the problem in what turned out to be the front left hub assembly.
Using the current weights on the wheels as the 12 O'clock position, I added a 1/4-ounce weight to the already-balanced wheel at that position and moved it around the clock. I road-tested the car after each movement until the vibration finally improved at the 8 O'clock position. Then I added another 1/4-ounce weight at that position, and the vibration disappeared completely.
Thanks to everyone for your suggestions, and if there are other people out there with the same problem, you might want to give this a try.