Refreshing RSS feeds: The Finer Points

I’m not too up to speed on RSS subtleties, but I need to implement a REALLY clean feed, basically a drop-dead look-a-like for a WordPress feed (shhhh). I’m looking at this:
http://rtfm.modx.com/display/ADDON/getResources.Building+a+RSS+feed

And it omits some of the finer points of a feed’s details, e.g.

* sy:updatePeriod -- e.g. hourly
* sy:updateFrequency -- e.g. "1", see http://www.feedforall.com/syndication.htm
* lastBuildDate -- a date of when the feed was last updated.

So discussion moves quickly to caching... I could add an "last_updated" TV to the RSS page, but how would I enforce this page to update based on the periodicity specified? Drupal has a crontab of sorts for scheduling system events... any ideas on how to do that? I could write a plugin that checks the RSS feed page’s "shelf life" time and sees if it needs to update it. I think that would work... would simply "touching" the page programmatically cause the cached version of that page to be discarded? Or would it be better to physically remove the cache file from core/cache/web/resources/ ?

Anyone know what format the system setting for server_offset_time is stored in? I think I can use that to populate the feed.

Any ideas? Has somebody already slayed this dragon?

Thanks!