From a glance at their archive, The Onion is still hosting articles from ten years ago. What in the world makes webmasters think it's okay to fail to maintain their mappings between URLs and live content?
Cool URIs don't change: http://www.w3.org/Provider/Style/URI
I feel your pain. I'm assuming you've already got something that works for you, but on the assumption that any problem worth mentioning is probably going to bite someone here eventually:
1) Don't try to manage 301s in server configs. You'll go insane.
2) Make a simple table mapping old URLs to new URLs. You can update this when you make a change that breaks URLs.
3) If you feel like you want to 404 or 501, and your server is not overloaded (if it is, pfft, let that spider eat a 404 and save resources for real people), check memcached to see if you've resolved this recently. If not, check the table and set the cache accordingly. You can then return control to the webserver to serve the cached error page or, alternatively, send the 301 to the proper page.
4) Give the end-user a quick page which returns what URLs are consistently getting 404ed and asks for a best page to 301 them to. Since that page is behind an admin login, you can make it as expensive as you darn well please -- for example, grepping the heck out of a large log file, goign row by row, and searching for a "best guess". You can let the users approve them with one click. (Last time I wrote one I put a little forecast of how much of the marketing budget was saved due to the users' diligence in assigning 301s. Five minutes of work, got me more pats on the back than most project which take 6 months. Apparently the admin staff was fighting over who got to do the URL corrections every day.)
Your users will love you, your database load will be low, your SEO will be awesome, and your crusty ol' sysadmin will not tear out your intestines and use them for a necklace the 47th time you ask for him to add a 301 to the config file.
To put this in perspective, I get approximately 40000 unique 404s from spiders every day for things that simply don't exist and never did.
In the real world, those tend to break somewhere around the second or third CMS upgrade.
"I have trouble believing any CMS author could fail to support them"
It almost sounds like you are assuming that The Onion has some sort of duty in maintaining ancient URIs. IMO, they should maintain their redirects as long as deem it necessary - there is a cost in doing so after all. Sure, the cost of not maintaining outdated URIs may or may not outweigh the cost of losing vistors, but that is for them to decide, not the peanut gallery.
I'm sure I'm not the only one that isn't so entitled that I wouldn't forgive The Onion if the links from my 2001 bookmark.htm export fail to land on the original content.