How do I convince UnmarshalJSON to work with a slice subtype?

I want byte slices that marshal and unmarshal in JSON using base64 RawURLEncoding instead of StdEncoding. There's no obvious way to do this through the encoding/json package, which is sensible, so I thought I'd create a subtype to do it.

type Thing []byte

Marshaling support is easy:

func (thing Thing) MarshalJSON() ([]byte, error) {
	if thing == nil {
		return []byte("null"), nil
	}
	return []byte(`"` + base64.RawURLEncoding.EncodeToString(thing) + `"`), nil
}

But Unmarshal not so much. I traced the encoding/json source, and came up with:

func (thing Thing) UnmarshalJSON(data []byte) error {
	v := reflect.ValueOf(&thing)
	if len(data) == 0 || data[0] == 'n' { // null
		v.SetBytes([]byte{})
		return nil
	}
	data = data[1 : len(data)-1]
	dst := make([]byte, base64.RawURLEncoding.DecodedLen(len(data)))
	n, err := base64.RawURLEncoding.Decode(dst, data)
	if err != nil {
		return err
	}
	v.SetBytes(Thing(dst[:n]))
	return nil
}

But yields a panic in the call to SetBytes():

panic: reflect: reflect.Value.SetBytes using unaddressable value [recovered]
    panic: reflect: reflect.Value.SetBytes using unaddressable value

I tried using a pointer to a slice, instead, which works (and doesn't require reflection), but causes other challenges elsewhere in my code that wants to work with slices instead of pointers.

So two questions, I guess:

1. Is this the best way to go about getting byte slices to marshal using RawURLEncoding?
2. If so, how can I convince my byte slice subtype to reference the data decoded from the RawURLEncoding format?

Use this code to unmarshal the value:

func (thing *Thing) UnmarshalJSON(data []byte) error {
  if len(data) == 0 || data[0] == 'n' { // copied from the Q, can be improved
	*thing = nil
	return nil
  }
  data = data[1 : len(data)-1]
  dst := make([]byte, base64.RawURLEncoding.DecodedLen(len(data)))
  n, err := base64.RawURLEncoding.Decode(dst, data)
  if err != nil {
	return err
  }
  *thing = dst[:n]
  return nil
}

The key points:

• Use a pointer receiver.
• Reflection is not needed to assign a []byte to a Thing.

playground example