The problem with extending the use of square brackets to cover sarcasm, tongue-in-cheek and incredulity is that square brackets traditionally mean "this context is being added and was not previously present in the quoted text". I.E. The Prime Minister stated, "Yesterday Mrs [George] Jones went to Hampshire." The editor of the top-most source is inserting "George" not to convey emotional meaning, but merely to convey contextual meaning within a quoted phrase.
Also, I believe the use of quotes to cover the cases I mentioned is already present in materials.
In a message dated 3/24/2009 8:18:44 A.M. Pacific Daylight Time, brewhaha@freenet.edmonton.ab.ca writes:
I think square brackets work better for your second case, because newspapers use them to correct grammar and insert context
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