Ok, the below is more or less the final result. Still going over a few things, but should have it ready for release tomorrow.
Both posts are aggregated away. Notice username links, etc.. are all disabled. Article and Page activity will still render basically as they normally would. Preview articles will ONLY render their title, intro text, and intro image with readmore rendering if there's full text. Articles also will not link if there is no readmore text.
The message is customizable with language string. It's also a template file for easy replacing. Since this flags activity rows as preview rows it's also possible to detect the preview state from auto actions and inject custom display behavior there as well. A CSS class is also added for easy custom styling of preview posts.
The parameter for this is very straight forward. Just give it a list of assets to allow previews for. Set to "all" to disable server side aggregation entirely. Supports wildcards too (e.g. gallery.%). It does not support substitutions.