Dan Mitsui at the Hieronymous blog posts some medieval art showing deacons and subdeacons holding torches during Mass at the Elevation. He asks if anyone has more information about this.
I don’t but, I did want to note that at Melanie’s parish, St. Louis, King of France, in Austin, the eight altar servers knelt in front of the altar holding huge candles during the Consecration and Elevation, but not during the rest of the Canon. I think it might be related.