Quote - PS - I understand the gimbal lock explanation, but I don't see how a mechanical problem like that relates to virtual axes as used in Poser.

What happens in Poser is due to the mathmatical consequences of the way rotation is done, there is no way to avoid the problem other than using a different representation for rotation internally (see -> Quaternions and Euler angles).

You can work around the problem by parenting the troublesome item to another and adjusting the Y rotation of the new parent item.