I'm hard pressed to find a solution better than the one you did, which is to provide technical guidance for the process in general and add a disclaimer (I'd prefer at the beginning of the answer vs. at the end) warning the developer to consider the legal and ethical ramifications of what they're doing.
At a previous job of mine this was actually a bit of an issue. We were aggregating public criminal records and in many cases that involved screen scraping on various jurisdiction websites. Some of the websites had statements indicating that the information was their property and that scraping was against the terms of service.
In that particular case, our company had a liability attorney on staff and he assured us that the statements are of no concern. The company made the decision to go forward with it against the advice of the developers. (Makes sense, he is after all the company's attorney. And the data is public records.) In one case it caused our scraper to be IP-banned by a server, but I don't know of any other problems that came about as a result.
Long story short, we as a community (both employees of and users of Stack Exchange) are in no position to offer legal advice of any kind. We aren't experts in law, we have no knowledge of the specific case, etc. Even if the site in question explicitly states that scraping is entirely illegal and violators will be killed to the fullest extent of the law, etc. that doesn't mean that the statement actually has any legal value.
The best we can do is offer technical advice and strongly urge the reader to seek legal counsel for non-technical concerns.
Edit: In re-reading your question, you make a very interesting point at the end. The idea of ensuring that respondents make such disclaimers. For that, I see two options for Stack Exchange:
- Have site-wide disclaimers (surely they already do, but maybe re-think the visibility of them) indicating that this isn't legal advice, etc., etc.
- Encourage (or at least not frown upon) users to edit other users' answers to include such disclaimers.
The former solution seems like the best. I'd hate to see the long-term results of the latter solution. Users would get upset that their answers are being filled with legal jargon when they don't see why it should. Answer meanings could be slightly changed by the edits, which does a disservice to the original respondent. Etc.