Hi Shammah,
This actually is exposed as a virtual, though in a roundabout way. A function marked as BlueprintNativeEvent gets an automatically generated virtual function with the same name with “_Implementation” appended to it. In this case we wanted to maintain backwards compatibility with the old blueprint callable CanJump() so had to add a CanJumpInternal() BlueprintNativeEvent. So in this case there is a:
virtual bool CanJumpInternal_Implementation() const
which you can override as you see fit. I’ll update the comments in the Character.h header to make it more clear. Sorry for the confusion!