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?