http://www.missionplanner.com.ar/
While it's rather outdated, it should be a decent start to mess around with offline.
To also elaborate, the 1st party offline architect never got off the ground. It was an idea that was mentioned in the 2007 survey, but never saw the light of day.
I thought I read something about a possibly 3rd party shareware offline editor for AE missions.
Is such a thing out there?
Or is it more a matter of people opening up the missions in a text editor and writing stuff into the codes?
Thanks!