That shouldn't be too hard actually. Can do it with coroutines, I am pretty sure:
IEnumerator RenderLeaderboardEnumerator() { // enable the loading spinner while (leaderboard_hasn't_loaded) { // update the loading spinner() * Time.deltaTime or something idk; yield return new WaitForEndOfFrame(); } // disable the loading spinner RenderLeaderboard() ;;; } ...
// in your OnEnable or wherever you are fetching: StartCoroutine(RenderLeaderboardEnumerator());
Something like that, I imagine. You will have to fill in a lot of the blanks though