http://forums.taleworlds.com/index.php/topic,78702.0.html
Shows how to do it with humans, similar should work with dragons. On attack suggestions you could see another thread, it suggests putting an invisible high armor, no attack rider on the creature, with a scripted death at the same time as the mount dies. You could also drop the creature in as a new soldier, but the scripting might be a bit harder.
mfberg