I'm curious to see what the CG shift from the muffler would be as well. I have a really hard time believeing that it would cause a trainer to go that unstable.

I've had and seen mufflers fall off several times over the years. Most of the time, the engine doesn't even stop running. (If it stopped, you're engine is set awfully lean to begin with, or your fuel system is marginal). And I've never crashed because of a muffler falling off. Espeically on something like trainer.