Can sunlight reflecting from a 400mm camera lens dazzle drivers about 300 feet away?

If what you need is a simple answer. Do not worry about a random flare. The flare would not be different from a flare from a random car in front of a passing car.

A more detailed answer.

1. Lenses are designed to get the light in, not bounce it. Even a flat filter in front of the lens lets most of the light pass, (let's say 99% light in and 1% reflected).

2. Most lenses without a filter, are convex, which means that the reflection is spread almost immediately.

3. The possibility of a flare of a flat lens hitting someone's eyes is minuscule, tiny, small. Besides that, to really annoy someone driving should be continuous light hitting for some seconds the eyes, so that means that you need to purposely move a mirror-like surface (which is not a lens) to follow your driver. Which is in fact very difficult. A small shake of your hands, a tiny angular change produces a big change in the distances.

4. Even if this small flare gets for a moment, the time of the day means that the sun is behind the driver, so the sun is illuminating the road ahead of the driver, making any flare insignificant.

Try one experiment at home. Get one of your son's lens filters, and play with it trying to hit that reflection in a specific target, Depending on the conditions you will not be able to find the reflected beam even at a couple of meters away.