as far as I know you are not really supposed to apply solvents to polycarbonate helmets as they can weaken the structure and cause the helmet to fail in an accident. Applying paint would be particularly doubtful as paint usually contains large amounts of organic solvents. Maybe if anyone knows of any water based fluorescent paint that would stick it could be OK but I have not seen any.

I think you could probably get away with applying reflective tape. Although officially I dont think you are supposed to do this either as the glue can also contain solvent that could theoretically eat away at the helmet.