Magically Enhanced Natural Attacks?

[shilsen]

Note: I would treat the Powerful Charge damage like sneak attack damage and not double the extra damage on a critical hit.

Why? The minotaur's Powerful Charge ability is just another kind of attack, so it should be able to critical. The Powerful Charge feat (found in the ECS, for example), on the other hand, provides extra damage dice on a charge, and by the critical rules that would not be multiplied.