Would "post pinning" apply to all the items from a specific source? How do you imagine it?
This stuff might prove useful: https://stackoverflow.com/a/22397092
We can partition the list of items before we sort them by dates (splitting them by the pinned sources), then sort both halves and join them together.
Essentially it will be a mockup similar to what I did in this post https://brettgilio.com/posts/2020-08-16-what-is-a-contributor.html. However, we can and should apply this to the entire webring, not just sharing a reply. A pinned post could have a push-pin icon next to the title to designate it as a pinned post (first in the order), where the rest of the most recent posts will be appended to the right of that pinned post. Does that make sense?
As far as implementation, some information would have to be specified, and I think a post URL would probably be the best choice, and we could do matching from there.