Dust could put a damper on laser indeed, depending on how dusty and how sensitive the laser is. You may have to experiment with it.
Ultrasound is out - it's in range but you can't get the accuracy.
Another solution I'm thinking of would be RF. Anything higher frequency than millimetre waves (i.e. wavelength < 1 mm) can measure distance with this accuracy and should be unaffected by dust. It would also require an appropriate reflector. I do expect such systems exist but have no ready links or an idea on cost.