Is it possible to use a IR proximity sensor as a dust optical sensor?
Dust sensors are available that basically use an IR light source (LED) and an IR detector (photodiode).  However they don't work by the dust breaking the beam as you would expect.

Instead they operate in a specially shaped chamber that prevents the IR light from reaching the detector under normal circumstances, but the detector is able to observe the area where the light is.

Then any dust that floats into that specially shaped chamber (often heated by a resistor to keep air circulating through it) reflects the IR light and the detector can see those sparkles and glints of light from the dust.

As you can imagine such an arrangement which relies on dust to be floating and moving to reflect the light right can't really be used when the dust is static on a windscreen.

However, if you are in an exceedingly dusty area it could be possible to position the light source one side of the windscreen at some distance and have the detector inside. Modulate the IR LED to some carrier frequency so that you can isolate it from the background light in the detector using a band-pass filter at the carrier frequency, and the strength of the signal received could indicate the reduction in light passing through the glass. How accurate this would be or reliable I cannot comment, but it may be something worth experimenting with.