Interesting, looking thru book the thing I am seeing under pg. 105, Ridden Monsters, Templates states "If you use a weapon or spell that uses a template, both the riders and the mount are automatically hit if the monster's base is touched by the template (note that this includes bouncing cannonballs!).
So all those test that use templates like Purple Sun...would require you to test both the riders and the mount...if the mount is destroyed the rider goes on foot, if the rider is destroyed you would roll on the Monster Reaction Table. Thinking that if it a draw a line test it would fall under the same ruling as a cannonball line.
It also states on pg. 105 under Ridden Monsters, "If a character model is riding a monster, it does not use rules for cavalry or monstrous cavalry, but rather those given here."
Those are my thoughts to the situation.