BJL: Doesn't HDR merge two or more images? That's different than dodging and burning, where you're adjusting one image...and NG's standard, whether it's foolish or not, is no HDR, and since it's their magazine, it's their call. They could equally say, "No red." This isn't fundamentally a matter of ethics, but a matter of institutional rules which may have some perceived connection to ethics.
I'll say again (for a third time) that you're all on a fool's errand if you're looking for ethics in a photographic image, which has no ethics, being inert.
Bob Fisher said: "I've stated, quite clearly, that when it comes to journalism and documentary there should be very minimal post-capture work done. I've said that when it comes to advertising/commercial I'm pretty liberal although I think the concept of truth in advertising has value. I've said when it comes to art, all is fair game."
Again, you're suggesting that the ethics lie in the product. It doesn't. When Andreas Gursky shot his famous photo of the Rhine, he post-processed it to remove some features he didn't like. This image was undoubtedly intended and accepted as art, and he made no secret of his post-processing. But if he'd presented the photo as being exactly naturalistic, then he, but not the photo, would have been a fraud, and the presentation unethical, whether it's art or not. Presented as it was, as a photo of the Rhine with post-processing, there's no problem, because post processing is fine in art, as long as you admit it. The ethics reside in the photographer, not in the image. In advertising, it's fine to show make-up on a gorgeous model, but it would be unethical to say that this makeup will make you as gorgeous as the model. And of course, any make-up company would instantly disavow any such intent, because otherwise they could be sued for deceptive advertising by a million unattractive customers who the make-up didn't help. What they will say is, here's a photograph of a gorgeous model who uses our make-up in an effort to make herself even more gorgeous, and that may very well be true, and might be be a reasonable objective for a buyer -- not to become the model, but to become a prettier version of herself.
When you ask the question "Is an altered image okay in any circumstance?" you're asking the wrong question -- an image is an object or a display, not a human being, so it can be neither ethical or unethical. That's why, when the cops find a child pornography image, they throw the photographer in jail, not the photograph.
The bottom-line question is, do you willingly and openly disclose what you've done with an image? If not, you're at least treading close to the line of unethical behavior. When the National Geographic moved a pyramid in a cover photo, it was unethical -- but it was the editor who was engaging in unethical behavior, not the photograph. Once again, the ethics lie with the person, not with the image.
There's also a somewhat related problem of "When and how much do I have to disclose?" Again, that does back to intent. If the intent is to deceive, then the action is unethical. If it's an obvious alteration -- say a photograph of John Kennedy with his hand on the shoulder of an adult Barack Obama, then you may not have to disclose because it's obviously a construct. If the line is questionable, then you should disclose.