Were there any studies supporting the information on brushing your teeth, or did that all come from authorities? The idea of orange juice softening tooth enamel sounds plausible, but implies that we shouldn't drink OJ before we eat either, right? A crunchy cereal would do at least as much damage as a tooth brush. Also, don't eat salad with vinaigrette dressing, since that is also acidic.
I did my own experiment to test the orange juice idea. All it needs are three cotton buds and some pH indicator papers. And of course some orange juice.
I dabbed some orange juice on an indicator paper: pH 3-3.5 as expected (mild acid). Then I swabbed some saliva from my teeth: pH 7-7.5, also as expected. Then I drank the orange juice, swishing it around my teeth to make sure it got everywhere, and as quickly as possible swabbed saliva from my teeth again: still pH 7-7.5.
I conclude that saliva is an effective enough buffer to neutralise any acidity from the orange juice almost as soon as it is swallowed. I wouldn't advise holding OJ (or other acid drinks like Coke) in the mouth for a long time, but why would anyone do that anyway?
Tests such as leaving an extracted tooth in OJ or Coke overnight are hardly realistic, as they simply leach enamel off dead tissue. A live mouth is very effectively pH regulated, so I'd think the effect is negligible.
Anyway, what is meant by "softening" the enamel? Is it being suggested that the outer layer of the tooth somehow becomes less hard for a short time, but if you don't brush it off it returns to its old, rock hard state. This seems implausible. Surely once calcium carbonate has been attacked by acid it has become CO2
and something like calcium citrate, which no longer makes hard enamel, so any effect would be irreversible.