Birdamor, you're completely correct with your information and there isn't one thing that you've said that I disagree with. Excessive accumulation of any supplement can possibly be fatal. It's the perfect representation of 'too much of a good thing'.
However, and not to take away anything from what you've said; but I did not read anything in that article that spoke of constant vitamin A supplementation. It just gave a list of vitamin A-rich foods that you can feed your bird to prevent Hypovitaminosis A. None of the information stated that you would supplement through anything but these foods, unless, the bird actually does get the disease.
I don't see any problem with regularly feeding vitamin A-rich foods as a preventative, but I do see the same problems that you do with using other means of supplementation. Vitamin A rich-vegetables are a more natural and much more safe method of treatment, and most likely won't have the side effects you're concerned about.