The problem is Mediawiki developers have to create yet another username on the labs instance, and will only get Echo notifications of replies to their topic when they return to the labs instance. This is another reason to put the LSG on mediawiki.org.
Maybe you can get the best of both worlds by having the discussion pages be #REDIRECTs to the Design/Living style guide talk page on mediawiki.org, and ask Danny to Flow-enable the latter. Or maybe the Discussion link in the sidebar itself could be hardcoded to always link to this one talk page.
We have a similar issue for the Data and developer hub prototype, T103785: Fix Discussion in Data and developer hub prototype.
Is there anything wrong with setting up the #REDIRECT. Not saying that the LSG shouldn't be on mediawiki, but as a stopgap solution, what problems could it cause? Or should we just do it?