Offer a free towel drying station. They can dry their own mirrors to their hearts' content.
I think you're aiming a little high here, which is fine. Honestly, mirrors drip after even the most thorough hand wash, so expecting better from your IBA is a lofty goal. In the Winter, when there's extra dirt, sand, and salt built up behind the mirror glass, water from the wash gets in there and it drips.