I’m inclined to say if a relational database works fine for our core Q&A content, it would be good enough for canonical posts too and if it wouldn’t work for canonical posts, it wouldn’t work for Q&A’s.
There are some valid concerns about using a relational database here, but canonical posts don’t seem to need to factor into that discussion.
But then I might also be missing why you think canonical posts would be so different from regular Q&A’s (in terms of underlying representation, not content).
Although I’m also of the mindset that it’s better to just get started with something and fix it later, which shouldn’t be too hard if your design is good and modular (which is where most people tend to go wrong, unfortunately), and I’m also probably a bigger fan of relational databases than I should be.