I accept that this question is similar to "do not do this", but please hear me out.
TeX is, at least in part, about beautiful typesetting. Now, although beauty is subjective, there are a number of commonly-agreed things that are generally considered ugly, and I believe that one of the measures of TeX.SX's "improvement of the Internet" is a question of beauty.
The idea of a beauty-alert tag would be to act as a flag to tell the questioner that what she is trying to do may result in something that is less than beautiful -- a sort of "beware of what you wish for".
Of course, I don't insist on that tag name. I did think it a bit better than my original blecch, ugliness, and ugly-alert ideas, as it looks positive, rather than negative, but I'm open to suggestions.
The tag description text would need careful crafting. Here's a starter:
What you are trying to do may be possible with TeX, but many, experienced, typographers think that the results are less beautiful than they otherwise might be. Please reconsider why you need to do this; but, of course, go ahead if you believe you're right.