Possible options for approaching this:
- Create a new table that records this info over time via a hook. The searches would only be able to go back to the day that we implement the table. This would probably be implemented as a MediaWiki extension. (Would this table collect info from all 800 wikis?)
- Try to use a cheap blame tool (similar to http://tools.wmflabs.org/xtools/blame/) that can track down exact revisions.
Also: verify with @Sadads that this approach would be helpful with the stakeholders he's working with.