I suspect @tophee that you and I are of a similar mind, since I seem to be following your threads around Meta lately. 
Indeed, and I think this discussion might get some more traction if retitled or posted anew with the focus more directly on the broader and (IMO) more interesting concept(s).
This seems like a potentially needless workaround. Surely there is (or could be) a more general way to do this.
There are at least 3 separate things being discussed here, and I think all three deserve consideration, arguably in separate threads.
First, Discourse being able to better present Onebox content from Hypothesis. This would be nice, but seems of somewhat limited use to my mind, provided you consider the more general case (outlined in point 3 below).
Second, Hypothesis being able to annotate Discourse forums. This may be a solved problem by now judging by some above responses, but if it’s not, it does seem like it would be nice if it worked. Arguably this is more important/useful than Hypothesis Oneboxing that allowed quoting of specific parts of a page, in light of…
Third, the more general idea of being able to quote anything on any page from any site using Discourse, which should not in my view be tied to Hypothesis specifically. I have actually found Discourse to have one of the very best implementations of quoting in any system, from the persistence and ease of quoting from multiple messages, to the optional expansion of context, and the quick jump to source, it’s an elegant implementation. It would really be notable, I think, if Discourse could do this on any site, not just the Discourse forum itself. Interestingly I think the case could be made that this might be especially useful in the context of Discourse for Teams, e.g. in research projects, competitive analysis, etc.
Obviously this would require either a browser plugin, or some kind of embedded page browser, or some other approach I’m not aware of, and it is likely to be non-trivial. But I think it has enough promise to be worth looking into. I think I could make a stronger case for it in its own thread if it seems justified…