JRG has asked a question you can link to, now you can do this:
If you see a question referencing a random .deb from the internets, edit it and add this notice:
Note: Installing packages from untrusted sources may harm your computer. Please consider finding an alternative, trusted source if you have to install this package.
If you know of one, definitely add a comment pointing to a more trusted source, such as an officially endorsed PPA (by the vendor) or the Ubuntu repositories themselves.
If you're sure your new source doesn't harm the quality of the answer, replace it altogether rather than adding a notice.
Please remember, this is only an "official policy" if you actually start doing it. Until then, I merely post this for the purpose of discussion.
But I'm championing this solution. Even though a couple of users will get disgruntled at the suggestion that their package is 'harmful', in the long term it'll help keep Ubuntu secure.