I think more having the modified AST feed back into the source code, or more accurately, the human-readable view of the actual source code. Like the comment example - imagine having the monkeypatch still in the comments module, but when you open the WallPost class, you can see the "has_many :comments" and a tag that takes you to the comments functionality.

Alternately, you can do the the reverse - someone writes the change in the more traditional manner, but you can view - and edit - the change-set as if it were a module of monkey-patches isolating the relevant concerns. Or any particular view of the program someone can think of that's useful.

I'd like to program like that.

ETA - Sorry for the accidental downvote; found a couple of your other comments to upvote.