If I grasp this correctly then I think that for this to work with OStatus it would be enough to provide a Salmon-endpoint that people could add to their sites metadata-files to point out that replies to stuff should be sent to that endpoint and when you receive it then you add it amongst the comment flows in the most suitable way. (Remember that replies can be atom-entries of every kind - not just simple comments that are posted but also other activities like "like", "favorite", RSVP etc - simple posts would be a good starter but the addition of more activity-types would be cool eventually)
Would love it if this became implemented - would probably add it right away to my GitHub Pages blog so that people could comment on those posts directly from a StatusNet-instance.
By Salmon-endpoint I'm meaning what is described here: http://salmon-protocol.googlecode.com/svn/trunk/draft-panzer-salmon-00.html#SRLR
There are some Python libs that might be possible to use: http://code.google.com/p/salmon-protocol/source/browse/#svn%2Ftrunk%2Flib%2Fpython
UX-wise this would probably work in such a way that platforms like StatusNet and Rstat.us would provide a URL for replying to an arbitrary URL and that when someone do so those platforms do a lookup towards that URL and check for a rel-salmon link-tag in the head-section of that HTML-page and sends Salmon-messages there. Such a URL provided by StatusNet and Rstat.us could be tied to pages through the use of Web Intents and a "reply"-intent - see http://webintents.org/ Web Intents are currently supported by Chrome. Of course a fallback on eg. a bookmarklet could also be provided by those services.
I was asked to comment, but I'm currently not entirely sure what I'm needed for. I'm about to walk out the door, and then go to Sweden, so I'm commenting so that I can be kept abreast via github notifications ;)