Sure, but not across a security boundary. Being able to override some library function such that running my text editor does $BADTHING isn't very interesting f

Sure, but not across a security boundary. Being able to override some library function such that running my text editor does $BADTHING isn't very interesting from a security perspective: if I have the capability to do that, I could also just run a program that does $BADTHING directly. Why bother with additional contortions to involve the text editor?