As the author of the cited answer, I can say that uppermost in my mind was that my answer might be useful to someone asked an almost identical question but without the meta-post restriction. I should also say that I was completely aware of the fact that this could be viewed as borderline for what is a clear and useful answer and that by posting it I was intending on people wondering about its suitability.
I hope that in this case, I was on the right side of the line. As anyone from MO (mathoverflow) will know, my views are pretty hardline on making sure that answers answer the specific question. But I think that this is one situation where the lower barrier-to-entry here compared to MO makes me more inclined to be relaxed; I hope that people will try to make their questions here focussed so that "I want to do A with T" is typical of the type of question we get. But I suspect that many people will not be aware that "S" is possible and so although their question specifically says "T", the answer with "S" will be fine. So that people who post answers aren't scared of posting such modifications, I would like to think that such answers are viewed as acceptable.
However, there clearly must be a line. It would be inappropriate to say "You asked about LaTeX, here's how to do it in ConTeXt".
If people agree with me that slightly out-of-the-box answers are acceptable, then for the actual line we can let the community decide. However, I would say that it's our responsibility (as early users) to set the tone that these, in moderation, are acceptable.
For anyone answering a question with an out-of-the-box solution, I would say that honesty is the best policy. Acknowledge that it may not answer the direct question, but that you hope it might be useful to others looking at that question.
As a general policy, I dislike doing things whose sole motivation is for "future questioners"; but I'm quite happy to make minor modifications in my behaviour to help these future ignoramuses (of whom I will probably be one!) if the gain is sufficiently large. My opinion was that this was one of those cases, but others may, of course, disagree!