I am sure if you asked your customers if they wanted their driveway / path gritted, they would say yes and you could even charge for it!!!
Traditional methods did not use the quantity of water that wfp uses, therefore this problem did not arrise. Explain this to the customer prior to doing the job, and explain that if they wish you can grit the areas that have the excess of water. If they refuse then the onus is on them to be careful of ice. If they say yes, I am sure they will be appreciative of your responsible precaustions and your thoughts for their safety.
Dunno if anywhere does biodegradable 'caution, slippery' signs that can be left?