I haven't actually heard of specific instances of this happening before (although it very well may have, of course). I like the idea, but it may be tricky to implement, and difficult to troubleshoot. Also, the idea of introducing one particular restriction into an otherwise unrestricted tool seems somehow prone to confusion.
What about possibly just adding a simple warning to the Journal Manager's User Home page, or on the page where they can log in as a given Editor, saying something to the effect that the "log in as" option should never be used to interfere with or change article content, and if content needs to be changed please contact Vanessa Gabler? If that may suffice, I can tell you where you'll need to make changes. Another option would be to include the caveat as part of a general policy to be handed out to all new (or even established) journals. Either way, the journal management team itself *should* ideally have a very good understanding of who can modify article content.