Reminding myself that this is information that nobody really needs to have at their fingertips, and it's just something that would be nice to be able to get to in some way, I simplified the whole thing down to a 1-line change that exposes the information as a tooltip on that little pre-composited icon that appears at the top-left of topic pages:
Search engines would love this sort of information. it would allow them to determine which posts are permanently archived so that they don't have to keep crawling them to check for updates.
Although I am not expecting Google et al to jump on this, but it is more useful for the hobbyist search engine implementations and designers out there to make indexing
slightly faster.