Um. Hand on a sec... on the bottom of the same page you link to:
Twitter still maintains a database of all
the tweets sent by a user. However, to
ensure performance of the site, this
artificial limit is temporarily in place
So maybe you can't do it now - but its not lost (yet).
Hmmm. Long ago I sent examples of the "NULL" issues for Twitter status URL's that I had bookmarked (possibly before favorites were available) as replies to the Twitter team -- not sure where any of that went but while a database is likely to exist, a guaranteed and representative content store may not be as likely.