I know this is a super stale thread, but I just wanted to voice my support for this approach. I’ll follow up with more official channels/recommendations in the future but I wanted to just ping here because it was your own idea.
My name is Ryan Levering and I handle structured data ingestion at Google (this guy). Discourse is sort of a snowflake in that it’s one of the only forum engines that uses DFPs at the root level for each comment in the thread. This makes it harder for us to figure out the type of the page AND to authoritatively detect the main post on the page.
We’re going to be recommending everyone use DiscussionForumPosting for the OP and then attach the rest of the responses to the original post as comment - Schema.org Property, exactly like recommended in the OP here. Other markup would be good as well (like more user interaction stats for instance), but that restructuring would go a long way toward making it easier to include Discourse forums in our features.