I am interested in developing a Onebox engine to neatly display/embed content from one of my own websites in a custom format - similar to the GitHub custom engines.
Assuming that I fork the Onebox repo on GitHub, and push all my custom code and changes, how would I then go about telling my Discourse instance to use my modified version of Onebox, rather than the official version?
The engine I am looking at developing is fairly custom, so I doubt that I will file a PR to include it back into the main codebase.
I did that to enable oneboxing of one of my own sites. It simply replaces the onebox with an iframe with a specific URL, where my website delivers a special embed-friendly view of the target site.
I pretty clueless when it comes to Ruby, so this may not be state of the art, but it might be enough to get you started
(Remember to also test what happens in notification mails containing the post.)
In that case, my approach has the advantage of needing only a tiny bit of ruby code, while most of the logic is happening on your website (which is responsible for producing the embed-view)