Based on this announcement, I don't think one HAS been able to efile a 1040-X (or a superseding return...) previously.

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WASHINGTON- The Internal Revenue Service  announced today that later this summer taxpayers will for the first time be able to file their Form 1040-X, Amended U.S Individual Income Tax Return, electronically using available tax software products.

Making the 1040-X an electronically filed form has been a goal of the IRS for a number of years. It’s also been an ongoing request from the nation’s tax professional community and has been a continuing recommendation from the Internal Revenue Service Advisory Council