Shouldn't be too big. Basically you could simply set up a part of prompt.arc to use a revisioning repository tool, and simply leave the working copy of the apps in each user's arc/app/ directory. When an wikiable app is edited, you can do a "cvs update" - "cvs commit" or "git pull" - "git add" - "git commit" or whatever your backend revisioning tool is. If a conflict occurs, send it back for editing, otherwise we succeed in the edit and return to prompt.
Basically you just wrap around the revisioning tool, which is effectively what a wiki with edit history does anyway.
