Is it okay to hang rosary beads from your rearview mirror?

If it is done as a statement of faith or for some other just purpose, hanging rosary beads on the rearview mirror would not violate canon law’s requirement that sacramentals be treated with due reverence:

Sacred objects, set aside for divine worship by dedication or blessing, are to be treated with reverence. They are not to be made over to secular or inappropriate use, even though they may belong to private persons (CIC 1171).