Register User with Services (JSON) with Custom Fields
Hope this will help you,

In Drupal 7 you can use the below JSON format to add custom fields(field_user_image)

{
    "name": "test",
    "pass": "password",
    "mail": "test@gmail.com",
    "field_user_image": {
        "und": [{
            "fid": "2"
        }]
    }
}


I have tried in my local and it is working fine.


Created new field UUID


Calling the Service using Firefox poster add-on


Code



  { 
    "name": "testuser123", 
    "pass": "testuser123", 
    "mail": "testuser123@gmail.com", 
    "field_register_uuid": 
      { "und": 
        [{ 
          "value": "65C7DCA4-8F03-473C-9E8F-08B3F29E91AE" 
        }] 
      }
  }


I can able to create new user with the custom field

References


How do I POST a new node using the Services module? 
Services module. How to construct the JSON data with session info for creating node?