The two different GMs I played Dragon Age on Roll20 with each had exactly the same idea. At the start of the game, select one of the dice and that one will always be the Dragon Die. One GM picked the first die, and the other picked the last die, and both times it worked well enough for us. I know it isn't fancy, but it works within the constraints that Roll20 allows, without needing to deal with the complexity of rollable tables. I imagine the same thing could be done with any system that has multiple dice colors to signify something important. Just requires someone to decide what number corresponds to which color. If you're like me and not very good completely terrible with tables, it is better than nothing.