I don't think it's possible. I did some research last year on how to accomplish this and the only way I could determine how would be modifying the UI for steemit/chainbb/etc.
The only way I could come up with how to do it would be dynamically generated images (used as signatures basically), but that doesn't work either because all images get cached via steemitimages and don't refresh. That means old posts would never see updates and it would basically take a snapshot the first time you used that specific URL.
If we could get a whitelist of domains that prevented this caching in the UI layer - then using images would be possible :)
Thanks. The way widgets do it in blogs such as Wordpress or Blogger is to insert code in the "head" that allows javascript, then in the "body" of the post insert the js script. That then generates the thumbnails/title just as in the post streams on steemit and chainBB.
So yes, it needs code inserted into the UI, even though the block appears inside a post. I can show you the code run by LinkWithin - I have a copy.
So could be achieved in chainBB even if not in Steemit?