The LUN endpoints do not support returning the serial number in hexadecimal form. However, you can retrieve the serial number through the REST API with "/api/storage/luns?fields=serial_number". You can then convert this returned string to to a hexadecimal string. (this is can be done by getting the hexadecimal value of each character in the string and putting them together to get the hexadecimal string)
While imperfect, this should get you what you're looking for!
Please let me know if you have further questions!
Daniel