I'm thinking about having someone stand in front of me holding an umbrella and seeing how badly it would mess up the umbrella or if it would look cool seeing the sparks bounce off of it. Sort of adding a different dimension to this cool light painting technique. If anyone else cares to try, have at it. It'll be a couple of days before I can try it.
Since we are brainstorming, if you had a way to be above and just behind the person holding the umbrella, then frame the shot to where the spinner was not in the shot, it would look like spark-rain. Wet down the umbrella first maybe?