Autoposting the links to the article and to the discussion in other site is heavily discouraged here. In general, bots are discouraged here. I's almost sure it will be downvoted heavily.
It doesn't say that explicitly in the guidelines, but I could see the spirit of that violating the norms of this site. OTOH, if the content was quality maybe it would be ok say if you only auto post top stories of the day / week with > a min# comments. Then the other site could have slow nuanced discussion and this site could build off that. What do you think?
IIRC is one of these implicit rules, you (or the OP) can ask the mods for more recommendations hn@ycombinator.com
Linking to other sites with a discussions is good. Every time there is airplane accident some link to an specialized forum (I don't remember which) and it's a good addition.
So linking to discussions with a lot of comments may fly under the radar. Even linking to a discussion with a single good comment may be welcome, but the comment must be really good. Perhaps it's even better to link to the good comment, instead of the whole discussion, unless most of the discussion is good. Anyway, except the first idea, the others look difficult to automate.