im not aware of any. but i may be able to give you a tip to help you find one. when searching, don't look for cases with charm loops. they just dont make them. you want a case with a loop for a LANYARD. i know some cases for iPods had this feature, although idk if any iPhone ones do. that loop would be the right size for your phone charms
although i agree, its too risky. id be afraid of damaging the phone