Note that edit pages and view pages possibly should be handled differently, or possibly not.
I can't see anything that edit pages could return other than 200 OK, the issue is with redlinks, since they don't actually link to what you would expect, the article (which doesn't exist and should return 404), they link to the edit page (which does exist, so should return 200). Could redlinks somehow return a 307 Temporary Redirect to the edit page, rather than linking there directly? The issue is determining when to redirect to the edit page and when to simply return a 404 - could the referer be used for that? If you've followed a link from the wiki, it should go to the edit page, if you've got there any other way (link from external site or typing in the url) you should get the 404. I'm not sure if treating a URL differently based on the referer would be against the standards.