A Zwei bag with a Klickfix seatpost mount is pretty close to a "large saddlebag that could also be used as a decently sized messenger bag". I saw the Zwei bags in Barcelona this past winter, and I was very tempted, despite having too many bags already! Wallingford says it's going to be selling Zwei -- maybe I'll break down when they're for sale in the US.

Zwei: http://www.iam2.com/english/main.html
Klickfix: http://www.klickfix.de/lenkeradaptere.htm
Wallingford: http://www.wallbike.com/