Steve,

No question about the folly of using doppler shift of light to detect low speeds or close ranges of objects.

Might the sensor be used for proximity on the basis of amplitude?

I've used single IR sensors to detect range when the object gives a good return signal. If the reflection characteristics are less diffuse and more directional in character multiple sensors can be applied, each having a different axis of symmetry, but overlapping fields of view. As an object moves through the field of view of multiple sensors its direction and position can be approximated.

Cheers,

Bruce