For what it’s worth, here’s what I ended up doing:
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On my blog, I have a <div>with an id offorum-excerpt, which is hidden withdisplay:nonebut contains the HTML that I’d like to show in the Discourse post. (I do this using some Jekyll / Liquid logic, but that shouldn’t really matter.)
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On my Discourse, I set the CSS selector for elements that are allowed in embedsto#forum-excerpt. Although the div is hidden on my actual page, the content shows up on the forum.
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I also uncheck Truncate the embedded posts.
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In the Embedded CSSsection, I give.buttona larger font. This is a small change but it makes the button to add a comment bigger.
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I have also customized the embed.continue,embed.start_discussion, andembed.imported_fromtext, which changes what shows up in the comments section on my website.
This means that I have full control over the HTML that shows up in the forum post. The HTML I give it is basically the equivalent of a OneBox- it’s a big thumbnail and a link to the main post.
This works pretty much perfectly for me, belated thank you for the help!