Is there any way to control how the oneboxing feature works with figcaptions (if I control both domains)?
Can you be more specific? What are you trying to accomplish?
I don’t think you can get access to the figcaption with a onebox - it’s not passed as an
og meta property.
Since you’re publishing the post through the wp-discourse plugin, you could probably customize the wp-discourse publish template to create what you’re looking for. It looks like what you’re doing now is setting the post’s permalink in the publish template, and that is creating the onebox.
Cool, thanks that is helpful.