I think you are onto a good point here; I think search engines have shun away from letting content authors write mark-up to trick their engines but with embedded widgets you kind of want a way to declare when something will expire.
There has been numerous times when I was searching on Google to find the content gone, looking at the cached version it was perhaps a sidebar of latest articles on stackoverflow to another article.
What I would however suggest is something more like:
In that this could be specified as a negative time to achieve complete irrelevance and any ISO date format was supported: http://en.wikipedia.org/wiki/ISO_8601
However obviously search engines would need to protect against abuse but I think that is only as complex as other things.