I don't feel TAv is identical to M + Auto-ISO because of how the Auto-ISO works differently. The Nikon system has it's disadvantage of not adapting itself to different focal lengths (to my current understanding, please correct if I am wrong). It won't care if you're using a 12mm ultra wide or a 300mm telephoto. If you have 1/125set for as a threshold, that's what it'll hold you to.
My primary use for TAv is for controlling consistent exposure within the same scene. I would not be using TAv to shoot a scene where there are great variance in amount of light. So yes, the Nikon's idea will work identical here.
The Nikon will have the bonus of being able to take pictures of scenes outside of your desired Shutter speed range to maintain EV without additional adjustments, but I would never be in TAv mode under those situations because I would be in a different mode to begin with.
In twinda1's example, I would be using Hyper Program with my Program Line set to Speed with Auto ISO enabled. Which I feel would give me more flexibility than Nikon's M + Auto ISO. In this mode, it would already default to using large apertures. If I realize the dof is too narrow, I adjust my aperture (front dial for me). If I realize I want faster shutter speeds I adjust my shutter (rear dial for me). Need a reset back to full auto? Green button.
Now if it was Auto focusing in twinda's example... I'd take just about any recent Nikons over my Pentax K10D