I like the idea, and I can imagine the look on my players faces as they come face to face with a dracolich with its controller and it starts behaving oddly in combat. I would most likey give the Dracolich Mongel a save every round, if it succeeds it wrests partial control for the round and puts itself in danger, "accidentally" catched itsallies in its breath weapon and otherwise fighting substandardly, croaking out the phrase "kill me, please kill me, stop the pain".
And then as the dragon "dies" the party hears the sound of several ghostly dragon voices saying "thank you"
Should make for an entertaining encounter.
Phaezen