But it already is easy. Even the lowly Berkeley Mail command has had this for about a decade. Any reasonable, modern mailer provides this feature. I prefer the Elm mailer . Piece 'o cake.
Any reasonable mailer provides it.
Whichever mailer you choose, please read the fine manual that comes with it. Unless you are stuck with some decrepit mail system, I bet you'll find it has a similar feature. If so, you easily can choose to direct your responses either to the original author or the entire list. Mauling the mail headers doesn't make it any easier. If you use a reasonable mailer, Reply-To munging does not provide any new functionality.