On a whim, refactored to support @replies too; I'm not sure if the 100/hour applies to each interface or any, but "two every 90s" is still only 80/hour. (Also not sure if the change particularly works, since the one @-reference to me wasn't the first one in the message. It's rather straightforward, though.)
It's run quietly for two days now; I did get one 502 Bad Gateway which is apparently "returned if Twitter is down or being upgraded"; looks like all of the 5xx codes can be handled by sleeping a little extra rather than outright failing, so I've added that too.